Chapter C: W. HEATON, F.I.C., F.C.S., Lecturer on Chemistry at the (21)

_Proc. of Ass._ The muffle, with the cupels properly arranged on the 'muffle-plate,' is placed in the furnace, and the charcoal added and lighted at the top by means of a few ignited pieces thrown on last. After the cupels have been exposed for about half an hour, and have become white-hot, the lead (see _below_) is put into them by means of the tongs. As soon as this becomes bright red and 'circulating,' as it is called, the specimen for assay, wrapped in a small piece of paper or lead-foil, is added. The fire is now kept up strongly until the metal enters the lead and circulates well, when the heat, slightly diminished, is so regulated that the assay appears convex and more glowing than the cupel itself, whilst the 'undulations' circulate in all directions, and the middle of the metal appears smooth, with a margin of litharge which is freely absorbed by the cupel. When the metal becomes bright and shining, or, in technical language, begins to 'lighten,' and prismatic hues suddenly flash across the globules, and undulate and cross each other, followed by the metal becoming very brilliant and clear, and at length fixed and solid (called the 'brightening'), the separation is ended and the process complete. The cupels are then drawn to the mouth of the muffle, and allowed to cool slowly. When quite cold, the resulting 'button,' if of silver, is removed by the pliers or tongs from the cupel, and after being flattened on a small anvil of polished steel, with a polished steel hammer, to detach adhering oxide of lead, and cleaned with a small hard brush, is very accurately weighed. The weight is that of the pure silver; and the difference between the weight of the alloy before cupellation, and that of the button of pure metal, represents the proportion of alloy in the sample examined. (See _below_.) In the case of gold, the 'button' has to undergo the subsequent operations of quartation, parting, and annealing, before it is weighed, as described under that metal.

_Assayer's weights, &c._ The materials used in assaying are accurately weighed in a balance of the most susceptible description; and the weights are given in terms of the 'notation' employed by assayers. The 'fineness,' 'richness,' or 'degree of purity' of gold is expressed in carats. Pure gold is spoken of as 24 carats fine; and any other sample containing in 24 parts only 12, 18, 22, &c., parts of pure gold, is said to be of as many carats fine. Every carat is nominally divided into 4 'assay-grains,' each assay-grain into 'quarters,' and each quarter into 'eighths' (= 1/32 carat), giving 768 "reports" for gold. On this system fractional alloys are commonly spoken of as of so many 'carats and thirty-seconds fine.' The real quantity taken for assay, technically termed the 'assay-pound,' is, however, very small, generally either 12 gr. or 6 gr., which makes each assayer's eighth-grain, or "report," equal to either the 1/64 or 1/128 gr. Troy, as the case may be. The nominal assayer's gold carat is 12 gr. The "journey-weight of gold" is 15 lbs. Troy (= 701 sovereigns = 1402 half-sovereigns).

The 'fineness,' 'richness,' or 'purity' of silver was formerly expressed in pennyweights; but is now generally reckoned in 1000ths, which admits of greater accuracy. Pure silver was said to be silver of 12 pennyweights." If it contained 1, 2, or 3 parts of alloy, it was termed "silver of 11, 10, or 9 pennyweights," as the case might be. Every assayer's pennyweight was nominally divided into 24 gr., and hence gave 288 fine grains, or 'reports,' for silver. The fineness of specimens containing odd grains was given in pennyweights and fine grains. The 'assay-pound' for silver, on this system, may be 24 Troy gr., when 2 real grains are equal to 1 'fine pennyweight,' and 1/12 real gr. equal to 1 'fine-grain.' In the decimal method pure silver is = 1000. The usual weight of silver taken for the 'assay-pound,' when the fineness is reckoned in 1000ths, is 20 Troy gr., every real grain of which represents 50/1000th of fineness; and so on of smaller divisions. The mint "journeyweight of silver" is 60 lbs. Troy (=3920 shillings, or a like value in other denominations).

_Ratio._ Cupellation, which is the distinctive and most important operation in assaying gold and silver, is founded upon the feeble affinity which these metals have for oxygen, in comparison with copper, tin, and other cheaper metals; and on the tendency which these latter metals have to oxidise rapidly in contact with lead at a high temperature, and to sink with it into any porous earthen vessel, in a thin, glassy or vitriform state. The conditions essential to the success of the process, and which are found in the precious metals, are--that "the metal from which we wish to part the oxides must not be volatile;" and that "it should also melt and form a button at the heat of cupellation; for otherwise it would continue disseminated, attached to the portion of oxide spread over the cupel, and incapable of being collected."[95]

[Footnote 95: Ure's 'Dict. of Arts, M., & M.,' 5th ed., i, 214.]

_Concluding Remarks._ The art of assaying requires very great care, skill, and experience, for its due exercise; and from the costliness of the precious metals, and their general employment for coin, jewelry, plate, &c., is of the utmost importance both to individuals and governments. Such is the extreme delicacy of the operation of cupellation that, without the requisites alluded to, it is more likely to fail than to give reliable results. An assay is thought to be good when the 'button' or 'bead' separates readily from the cupel, has a round form, with a brilliant upper surface, and the lower one granular and of a dead metallic lustre. When the upper surface is 'dead' and 'flat,' too much heat has been employed; and in the case of silver, some of the metal may have been lost by fuming or absorption. When the bead adheres to the cupel, or is spongy, variegated, or has scales of litharge still adhering to it, either too little heat has been used, or the process has been stopped before the assay was complete. The remedy is re-exposure to heat in the cupel, adding a little powdered charcoal or a few small pieces of paper, and continuing the heat until the metal 'brightens' and 'circulates' freely. The lead employed must be absolutely pure, or that technically called 'poor lead,' and, for this purpose, is commonly prepared by the reduction of refined litharge mixed with some carbonaceous matter, by heat; but, according to the late T. H. Henry, "lead reduced from the litharge of commerce usually contains from 10 to 15 _dwt._ of silver per ton." These remarks apply equally to gold and silver.

The process of assaying by the cupel, however skilfully conducted, gives much less accurate results, especially with silver, than the method of chemical analysis, often termed 'humid' or 'volumetrical assay,' whilst it is, in all cases, much more troublesome and expensive, and with compounds containing only small quantities of the precious metals, is not to be depended on. See GOLD, SILVER; also CARAT, CUPELLATION, PARTING, LIQUATION, QUARTATION, REFINING, &c. (and _below_).[96]

[Footnote 96: Those desirous of further information on the subject than that contained in this work, are referred to Mitchell's 'Manual of Assaying,' and to the various memoirs of Gay-Lussac, Chaudet, D'Arcet, Tillet, Brande, Ure, Henry, and others.]

=Assay of the Touch.= The fineness of JEWELRY, and of small quantities of GOLD which it is either impossible or inconvenient to assay according to the usual method, is generally determined by means of touch-needles and touch-stones. The former are made in sets, containing gold of different degrees of fineness, and differently alloyed with copper and silver. The latter are usually of black basalt; but pieces of good black pottery answer the purpose very well. The mode of using them is to mark the stone with the sample under examination, and to compare its appearance, hardness, colour, &c., with that produced by one or more of the needles. When the two are similar, the quality or 'fineness' is considered to be the same. The marks are then further examined by heating the 'touch-stone' to redness, and moistening the strokes with aquafortis, when the appearance resulting from oxidation, &c., differ according to the nature and quantity of the alloy. A nearly similar method is sometimes adopted with SILVER; but the characteristics are scarcely so distinct with the metal. (See _above_.)

=Hu'mid Assaying, Humid Assay.= Terms applied to the estimation of the quantity of gold and silver in ores and alloys in the moist way, more especially by the method known as volumetrical analysis. See GOLD (ESTIM.), SILVER (Estim.), VOLUMETRICAL ANALYSIS, &c.

=ASSIMILA'TION.= [Eng. Fr.] _Syn._ ASSIMILA'TIO, L.; ANEIGUNG, VERÄHNLICHUNG, &c., Ger. In _physiology_, the conversion of food into nutriment, and finally into the substances which compose the bodies of animals and plants; the function of nutrition.

=ASTHEN'IC.= _Syn._ ASTHEN'ICUS, L.; ASTHÉNIQUE, DÉBILE, Fr.; SCHWACH, Ger. Weak; debilitated. In _pathology_, an epithet of diseases (ASTHEN'IC DISEASES) accompanied by great and well-marked debility.

=ASTHEN'OPY.= _Syn._ ASTHENO'PIA, L. In _pathology_, incapacity to keep the eyes fixed on near or small objects for any length of time without confusion of vision. The common causes are over-exertion of the eyes, particularly by artificial light, or by a very brilliant one, or during convalescence; congestion of the ocular vessels; debilitating discharges or indulgences; and general nervous debility, however produced. It "appears to consist in weakness of the apparatus by which the eye is adjusted for the vision of near objects;" and along with this "there is an irritable state of the retina, connected in some manner with a tendency to internal congestion of the eyes."[97] The _treatment_ may consist of rest to the eyes, and ablution of them in cold water, with such other efforts to restore their tone and the general health as are noticed under AMAUROSIS. The prospect of complete cure, when the cause is not removable, is unfavorable; but even when confirmed the disease is not likely to end in blindness. The use of convex spectacles of very low power will generally be found serviceable. See EYE, SPECTACLES, VISION, &c.

[Footnote 97: T. W. Jones, 'Defects of Sight,' Lond., 1856; p. 82.]

=ASTH'MA= ([)a]st'm[)a][98]). [Eng., Ger., L., Gr.] _Syn._ ASTHME, Fr.; ENGBRÜSTIGKEIT, Ger. In _pathology_, a well-known disease coming on by fits, and characterised by shortness and difficulty of breathing, accompanied by a wheezing sound, cough, stricture and tightness of the chest, with other like symptoms. These gradually increase until the patient can no longer remain in a recumbent position, being, as it were, threatened with immediate suffocation; and they generally terminate, after the lapse of a few hours, in copious expectoration. The attack usually commences towards evening, and the symptoms increase in urgency during the night--often occurring suddenly after the first sleep--until at length, on the approach of morning, a remission takes place, and, in all probability, the patient, worn out and exhausted, falls into a sound sleep. On awaking in the morning he still feels the 'tightness' at the chest, breathes with some difficulty, which is increased by moving, and cannot lie in bed unless his head and shoulders are greatly raised. After a repetition of the fits for some nights, they at length moderate, and after more considerable remissions, pass off at last, leaving the patient in his usual state of health for a time, or until fresh exciting causes produce a return of the disease. For an evening or two previous to the fit the patient generally feels drowsy, indolent, and low-spirited, and experiences a sensation of fulness about the stomach, with headache, general uneasiness, and indigestion--these are the premonitory symptoms.

[Footnote 98: Asth'ma (with th fully sounded), as given by Knowles, is difficultly pronounceable, and is now obsolete.]

Asthma is principally confined to the later periods of life, and appears in many cases to be hereditary. It is generally severest in the heat of summer, or in the foggy or damp or windy weather of winter. The fits vary in duration from two to several hours. Sometimes copious expectoration commences early, which has led to the division of asthma, by nosologists, into two kinds--dry, nervous, or spasmodic asthma (ASTHMA SIC'CUM, L.) and humid a. (A. HU'MIDUM, L.).

The exciting causes of asthma are exposure to sudden changes of temperature, particularly from heat to cold; unwholesome effluvia, hard drinking, heavy meals, indigestion, violent exercise, and cold, damp, foggy, and sometimes windy weather.

_Treatm._ A dry, warm, and airy situation as a residence should, if possible, be sought. The use of flannel next the skin, and tepid or warm bathing is also advantageous. The bowels should be kept regular by mild aperients, and the stomach preserved in order by the adoption of a light and wholesome diet; particularly avoiding excess in either eating or drinking. The severity of the paroxysm may be generally lessened by adopting the sitting posture, and inhaling the vapour of hot water or of an infusion of chamomile. Small doses of camphor, ether, and opium, frequently repeated, may also be tried. The inhalation of the vapour of a little tar liquefied by heat is said to often produce considerable relief. The fumes arising from the slow combustion of porous paper dipped in a solution of nitre, and dried, have also been recommended. "The fumes of a piece as big as one's hand being placed on an earthenware plate, and ignited, presently become sensible throughout the room; and within a quarter of an hour their influence in many cases is rendered evident, in clearing the passages and gradually opening the air tubes." "Of calming vapours that of chloroform is, however, the one likely, in respect of its soothing power, to supersede all others. Inspired in moderate quantity, far less than is requisite to produce general insensibility, it has been found of singular efficacy in allaying, at once, the spasmodic distress of an asthma-fit. But it is a remedy too potent and subtle to be entrusted to the discretion of the patient himself." (Dr Watson); unless, indeed, he well understands its properties and nature, and has some friend near him to restrain his using it too freely--a thing he is, unfortunately, often tempted, by the urgency of the symptoms, to do. "Bleeding is an imprudent operation in every species of asthma" (Dr Bree); and has often proved highly injurious, especially in elderly persons. It is only in full plethoric habits, or when the paroxysms are very severe, and attended with signs of congestion of the lungs and brain, indicated by lividity of the countenance, stupor, extreme dyspn[oe]a, &c., that blood should be taken; and then only by 'cupping' between the shoulders, or by leeches to the chest. Emetics and active purgatives must also be avoided during the paroxysm; at which time costiveness may be best removed by an aperient clyster containing assaf[oe]tida. At other times, emetics (of ipecacuanha) and diaphoretics, followed by mild purgatives, may be administered with advantage; indeed, an emetic, taken a few hours before an impending fit, will frequently prevent its accession. Dyspeptic symptoms must be treated in the usual manner. "Chalk and opium will astonish the asthmatic, by the excellence of their effects when the irritation proceeds from dyspepsia of the first passages only." (Dr Bree.) The same authority also states that vinegar, separately administered, counteracts the flatulence and distension of the stomach.

Various other remedies have been recommended for asthma; among which are the smoking of tobacco and stramonium. In using the latter herb, the root and lower parts of the stem are chopped up and placed in the bowl of a common tobacco-pipe, and a few whiffs are occasionally taken. Drinking at the same time should be avoided. Lately _lobelia inflata_ (Indian tobacco) has been highly extolled in asthma, in doses of--tincture, 20 or 30 drops, to 2 teaspoonfuls--powder, 5 to 15 or 20 gr.; taken at the commencement or shortly before the accession of the fit, and repeated after the interval of an hour, if nausea or expectoration does not intervene. Sir John Floyer is said to have been cured of an asthma of 60 years' standing, at the age of 80, by the constant use of very strong coffee. Sir John Pringle adopted the same remedy with great success. He remarks, "One quality occurred to me which I have observed of that liquor (coffee), confirming what you have said of its sedative powers. It is the best abater of periodic asthma which I have seen. The coffee used ought to be the best Mocha, newly burnt, and made very strong immediately after grinding it. I commonly order an ounce for one dish, which is to be repeated with fresh coffee after the interval of a quarter or half an hour; and which I direct to be taken without milk or sugar."[99]

[Footnote 99: 'Letter to Dr Percival.']

Very recently cigars and cigarettes of _datura tatula_, Linn.--a peculiar species of stramonium--have been prepared by Messrs Savory and Moore; and are strongly recommended by Drs Watson, Latham, Fergusson, and many other physicians of eminence, as the very best remedy yet introduced for asthma.

A change of diet and habits, and particularly a change of residence, will often produce a marked improvement in asthmatic patients, and even effect a cure, when medicines have failed. The use of bark and bitters, or mild chalybeate tonics (when not contra-indicated), tends to improve the tone of the system, and may be adopted, in nearly all cases, with perfect safety. See BATH, AIR (Compressed), CIGARS, DATURA, &c.

_Treatment for Horses._ Ether and belladonna; chlorodyne; inhalation of chloroform; or amyl nitrite; subcutaneous injection of morphine or atropine; arsenic; and regular digestible diet.

=Asthma, Grind'er's.= See MELANOSIS.

=ASTHMA CURE.= 1. (Dr Aubrée, Ferte Vidame, Eure et Loire, France.) Decoction of senega (10 parts of the root), 250 parts; iodide of potassium, 50 parts; extract of opium, 4 parts; simple syrup, 500 parts; weak spirit, 200 parts. Coloured with some cochineal tincture. (Hager.)

According to a later analysis by Schröppel, this remedy is thus composed:--Iodide of potassium, 9 parts; French lactucarium, 1 part; water, 288 parts; simple syrup, 48 parts; chloric ether, 1-1/2 part.

2. (Kubale, Klitschdorf, near Bunzlau.) This is a solution of iodide of potassium, bromide of potassium, and sugar in water, strongly coloured with a cochineal tincture containing alum. It is supplied in six bottles, numbered 1 to 6, No. 1 being the weakest, and No. 6 the strongest in the iodide and bromide. In No. 3, for example, we found:--Iodide of potassium, 5 grms.; sugar, 2-1/2 grms.; alum, 1/3 grm.; cochineal colouring matter, 1/2 grm.; water, 200 grms. (Hager.)

=ASTHMA TEA= (Dr Orleïn). Recommended for difficulty of breathing, dry coughs, loss of sleep, loss of appetite, &c. Liquorice, 8 parts; marshmallow root, 6 parts; Iceland moss, 5 parts; a sort of buckbean, 2 parts; horehound, 2 parts. (Schädler and Selle.)

=ASTHMATIC PASTILLES= (S. Kittel's, now Daniel White & Co., New York). Set fire to the pastilles and inhale the smoke. An analysis found in 100 parts:--Nitrate of potash, 20·1 parts; impure resin of scammony, 3·5 parts; gum and sugar, 35· parts; charcoal, plant-stems, and leaves, 40·7 parts. (Dr Fleck.)

=ASTRIN'GENT= (-tr[)i]nje'-). [Eng., Fr.] _Syn._ ASTRIN'GENS, L.; ZUSAMMENZIEHEND, Ger. That straitens or causes wrinkling or constriction. In _pharmacology_, an epithet of substances or agents (ASTRIN'GENTS; ASTRINGEN'TIA, L.) which constrict animal fibre and coagulate albuminous fluids, and thereby obviate relaxation and check excessive secretion or discharges. In modern use, the word, both as an adj. and subst., is chiefly applied to internal remedies, those of a like character, employed externally, being usually termed 'styptics,' 'desiccants,' &c.

The principal astringents are--alcohol, alum, chalybeates (generally), sulphate of copper, sulphate and perchloride of iron, acetate and diacetate of lead, lime, bichloride of mercury, nitrate of silver, vegetable astringents (see _below_), acetate, carbonate, chloride, oxide, and sulphate of zinc, &c. See DESICCANTS, STYPTICS, TONICS, &c.

=Astringents, Min'eral.= See ASTRINGENT (_above_).

=Astringents, Veg'etable.= Of these the principal are--alkanet, bistort, catechu, the cinchona barks and their alkaloids, dragon's blood, French or red rose, galls, kino, logwood, mastiche, oak-bark, red sanders wood, rhatany, tormentil, tannic acid, gallic acid, and areca nut. (See _above_.)

=Astringent Prin'ciple.= A term formerly restricted to tannin; but now commonly applied to the astringent matter of any vegetable.

=ATMOM'ETER.= _Syn._ ATMIDOM'ETER; ATMOM'ETRUM, &c., L.; ATMOMÈTRE, &c., Fr. In _chemistry_ and _meteorology_, an instrument for measuring the rate of evaporation from a humid surface. It is of very simple construction, and possesses some practical value. It consists of a long glass graduated tube divided into inches, having attached to the bottom a hollow ball made of porous earthenware, similar to that used in water bottles. When used, water is poured in at the top until it rises to the zero point of the scale. The outside of the porous ball being always covered with dew, the more rapidly the evaporation takes place, the more quickly will the water fall in the tube.

=AT'MOSPHERE= (-f[=e]re). _Syn._ ATMOSPHE''RA, L.; ATMOSPHÈRE, Fr.; ATMOSPHÄRE, DUNSTKREIS, Ger. Primarily, a 'vapour-sphere,' appr., the assemblage of respirable gas and aëriform vapours which surround the earth; fig., any surrounding medium or influence.

_Comp., Chem. prop., Pur., Uses, &c._ See AIR (Atmospheric).

_Mechanical properties_ of the atmosphere:--

COLOUR:--The prevailing colour of the atmosphere is blue; at considerable elevations this blue tint is lost, and the sky appears deep black. The prevalence of blue is referred to the greater facility with which the blue and violet rays are reflected, whilst the glowing tints of morning and evening are conceived to arise from the red rays possessing greater momentum than the other rays of the spectrum.

DENSITY:--The density of the atmosphere diminishes with the distance from the earth's surface, and this is the duplicate ratio of the altitude. Thus, if at a given altitude the density of the air is only one half what it is at the level of the sea, at twice that elevation it possesses only one fourth that density. On this fact depends the application of the barometer to the determination of the elevation or depression of any point above or below the level of the sea, taken as a standard.

_Density of the Atmosphere at Different
Elevations._ By Prof. GRAHAM.

+----------------------+-------+-------------+
|Height above the level|Volume |Height of the|
| of the Sea in miles. |of Air.| Barometer. |
+----------------------+-------+-------------+
| 0· | 1 | 30 |
| 2·705 | 2 | 15 |
| 5·41 | 4 | 7·5 |
| 8·115 | 8 | 3·75 |
| 10·82 | 16 | 1·875 |
| 13·525 | 32 | ·9375 |
| 16·23 | 64 | ·46875|
+----------------------+-------+-------------+

HEIGHT, &c.:--If the density of the air were uniform throughout its whole extent, the height of the atmosphere, measured by a corresponding column of mercury, would be barely 5-1/4 miles. As, however, its density decreases with the distance from the earth's surface, its real height must be considerably greater. Kepler found that the reflection and refraction of the sun's rays by the atmosphere, producing twilight, ceases when that luminary descends 18 degrees below the horizon, whence it is calculated that the atmosphere cannot have a greater altitude than 45 miles. On the other hand, there is reason to believe that it cannot be much less than this sum. "With a good air-pump air may be rarefied 300 times; supposing this to be the utmost limit to which rarefaction can be carried, the atmosphere would still extend to an altitude of above 40 miles." Whether, in a state of extreme tenuity in which its grosser properties are lost, it extends indefinitely into space, was formerly a subject of controversy. That its boundaries are limited, and that it belongs exclusively to our earth appears almost certain. "We are warranted in concluding that the atoms of air are not infinitely divisible, and consequently that the atmosphere has a limit; and the limit must be situated at that height above the earth where the gravitation of the atoms is just equal to the force of their repulsion."[100] Under ordinary circumstances the mercury of the barometer falls about one inch for every 1000 feet of elevation.

[Footnote 100: Brande's 'Dict. of Lit., Sci., & Art.']

PRESSURE:--The weight or pressure of the atmosphere is shown by the rise of water in the barrel of the common 'lifting pump' and the suspension of the mercurial column in the tube of the barometer. The last affords a ready means of determining the actual pressure of the air, the column of mercury, and the column of air by which it is suspended, resembling two weights in equilibrio, at the opposite extremities of the same balance. The mean height of the barometer at the level of the sea, in England, is 28·6 inches (= about 33-1/2 feet of water); and as a cubic inch of mercury weighs 3425·92 gr., or ·48956 _lb._, it follows that the weight of a column of mercury whose base is a square inch is 14·6 _lbs._ avoirdupois. The pressure of the atmosphere is not merely downwards, but is equally diffused in all directions, and exerts a most powerful effect in the economy of organic beings. On the surface of the body of an adult of ordinary size (say = 15 sq. feet, or 2160 inches), it amounts to the enormous weight of 31,536 _lbs._, which is not sensible, only because it is balanced by the force of the elastic fluids in the interior of the body. Were this equilibrium to be suddenly destroyed, the consequence would be, either that the body would be instantly torn to pieces with explosive violence, or that it would be crushed under the overwhelming weight that would suddenly fall upon it. Even the comparatively slight variations of atmospheric pressure which occur with changes of wind, weather, and season, exercise a perceptible effect on the functions of life.

_Mean pressure of the Atmosphere at the level of the
Sea, in different latitudes, at 32° Fahr., expressed in
inches of mercury_.

-----+------------+----------+-----------+-----------+------------
Lat. | Height | Lat. | Height | Lat. | Height
| (inches). | | (inches). | | (inches).
-----+------------+----------+-----------+-----------+------------
0° | 29·930 | 40° | 30·019 | 54-1/2° | 29·926
10 | 29·975 | 45 | 30·000 | 60 | 29·803
20 | 30·064 | 49 | 29·978 | 64 | 29·606
30 | 30·108 | 51-1/2 | 29·551 | 67 | 29·673
-----+------------+----------+-----------+-----------+------------

TEMPERATURE:--The temperature of the atmosphere, independently of changes arising from variations of latitude and season, diminishes, like its density, with its elevation. In general, every 100 yards of ascent causes the temperature to fall 1° Fahr. See AIR (Atmospheric), EPIDEMICS, VENTILATION, &c.

=Atmosphere=. In _engineering_ and _pneumatics_, the pressure of a column of mercury at 0° Cent. or 32° Fahr., which is 76 centimètres or 29·9218 inches high, at the mean level of the sea in latitude 45°, taken as a standard of that exerted by other elastic fluids. In practice this is assumed to be 15 _lbs_. to the square inch, under a barometrical pressure of 30 inches. Thus, steam or air condensed so as to exert a pressure of 30 _lbs_. per sq. inch is said to be of two atmospheres; at 45 _lbs_., of three atmospheres, &c.

=AT'OM (-[)u]m). Atomic Weight, Atomic Theory=. _Syn_. AT'OMUS, L.; ATOME, Fr.; ATOM, UNTHEILBARE THEILCHEN, Ger.

ATOMIC WEIGHT. When the elements unite chemically, they invariably do so in the proportions by weight represented by the numbers attached to them in the following table, or in multiples of these proportions. Dalton accounted for this law by supposing that the constituent particles of matter are indivisible, and believed that, if it were possible to place such particles in the balance, their relative weights would be found to correspond with the numbers given in the table.[101] In other words, the term _atom_, which is derived from the Greek [Greek: atomos], indivisible, is applied in modern chemistry to the smallest quantity by weight of an element which is capable of existing in a chemical compound, hydrogen being taken as unity.

[Footnote 101: Strictly speaking, Dalton, the inventor of the Atomic Theory, did not adopt the precise numbers given in the table, but others, which, however, bear a very simple relation to them.]

--------------------+------------+-----------+------------
Name. | Symbol. | Atomic | Atomic
| | weight. | volume.
--------------------+------------+-----------+------------
=ALUMINUM= | Al | 27·5 |
ANTIMONY | Sb | 122 |
ARSENIC | As | 75 | 1/4
BARIUM | Ba | 137 |
BISMUTH | Bi | 208 |
BORON | B | 11 |
=BROMINE= | Br | 80 | 1
Cadmium | Cd | 112 | 2
Cæsium | Cs | 133 |
=CALCIUM= | Ca | 40 |
=CARBON= | C | 12 |
Cerium | Ce | 92 |
=CHLORINE= | Cl | 35·5 | 1
CHROMIUM | Cr | 52·5 |
COBALT | Co | 58·8 |
=COPPER= | Cu | 63·5 |
Didymium | D | 96 |
=FLUORINE= | F | 19 | 1
Glucinum | G | 14 |
GOLD | Au | 196·7 |
=HYDROGEN= | H | 1 | 1
Indium | In | 74 |
=IODINE= | I | 127 | 1
IRIDIUM | Ir | 198 |
=IRON= | Fe | 56 |
Lanthanum | L | 92 |
=LEAD= | Pb | 207 |
Lithium | Li | 7 |
MAGNESIUM | Mg | 24 |
=MANGANESE= | Mn | 55 |
=MERCURY= | Hg | 200 | 2
Molybdenum | Mo | 92 |
NICKEL | Ni | 58·8 |
Niobium | Nb | 97·6 |
=NITROGEN= | N | 14 | 1
Osmium | Os | 199 |
=OXYGEN= | O | 16 | 1
PALLADIUM | Pd | 106·5 |
=PHOSPHORUS= | P | 31 | 1/4
PLATINUM | Pt | 197·4 |
=POTASSIUM= | K | 39 |
RHODIUM | Rh | 104 |
Rubidium | Rb | 85·5 |
Ruthenium | Ru | 104 |
Selenium | Se | 79 | 1
=SILICON= | Si | 28·5 |
=SILVER= | Ag | 108 |
=SODIUM= | Na | 23 |
STRONTIUM | Sr | 87·5 |
=SULPHUR= | S | 32 | 1
Tantalum | Ta | 137·5 |
Tellurium | Te | 128 |
Thallium | Tl | 204 |
Thorium | Th | 231·5 |
TIN | Sn | 118 |
TITANIUM | Ti | 50 |
TUNGSTEN | W | 184 |
URANIUM | U | 120 |
Vanadium | V | 51·2 |
Yttrium | Y | 68 |
=ZINC= | Zn | 65 | 2
Zirconium | Zr | 90 |
--------------------+------------+-----------+------------

ATOMIC VOLUME. The volume or space occupied by the atomic weights of gases at a temperature of 60° F., and under a pressure of 30 inches of the barometer, compared with that occupied by one part by weight of hydrogen under the same conditions.

In the same table the most important elements are distinguished by the largest type, those next in importance by medium type, and those of rare occurrence, or of which we know but little, by the smallest type.

=ATOMIC WEIGHTS.= See ATOM.

=ATON'IC.= _Syn._ ATON'ICUS, L.; ATONIQUE, Fr.; ATONISCH, SCHLAFF, Ger. Weak; debilitated; deficient in tone or strength. In _pathology_, applied to diseases or conditions of the body (ATONIC DISEASES; ATONY) in which debility is the leading feature. In _pharmacology_, ATONICS are agents which relax or lower the tone of the system.

=AT'ONY.= _Syn._ ATO'NIA, L.; ATONIE, &c., Fr., Ger. In _pathology_, loss of tone, relaxation, morbid diminution of vital energy or power; commonly applied to debility of any kind.

=AT'ROPHY= (-fe). _Syn._ ATRO'PHIA, L.; ATROPHIÉ, &c., Fr.; ATROPHIE, Ger. In _pathology_, wasting or emaciation, with loss of strength, and unaccompanied by fever or other sensible cause; defective nutrition; decline.

_Classif., Causes, &c._ It is either local, as in the case of a limb which is small, imperfectly developed, or withered; or general, affecting the whole body. GEN'ERAL ATROPHY appears to depend on deficient nutrition, arising from a want of due balance between the functions of assimilation and absorption, or from profuse evacuations draining off the materials necessary for the support of the body. In the former case only may it be regarded as an independent disease. LO'CAL ATROPHY commonly arises from some cause which lessens the normal circulation of blood in the part; or from a diminution of the nervous influence, as in paralysis. General atrophy is most frequent in infancy, childhood, and old age. In the first two it may be often traced to bad nursing, worms, or a scrofulous taint; and not unfrequently to continually inhaling impure or damp air. In adults, the causes are impaired digestion and imperfect action of the chyliferous organs, and sometimes diseased action of the liver. In many cases it results from the use of tobacco.

_Treatm._ This consists in a close attention to diet (which should be liberal and nutritious), exercise, clothing, ventilation, warmth, &c., with gentle stimulants, and chalybeate tonics where not contra-indicated; and, in the case of adults, the moderate use of pure generous wine or malt-liquor. Among special remedies, both in this disease and anæmia, may be mentioned pure sweet cod-liver oil, which seldom fails to arrest or greatly retard the progress of the disease, and in very many cases effect an entire cure. When this affection is symptomatic of any other disease, as worms, stomach or liver complaints, &c., the removal of the latter must of course be first attempted. See ANÆMIA, CHLOROSIS, TABES, &c.

=ATRO''PIA= (tr[=o]pe'y'[)a]). C_{34}H_{23}NO_{6}. [L.; B. P.] _Syn._ AT'ROPINE (-pin; sometimes atro'p[)i]ne[double-dagger]), Eng., Fr.; ATROPI'NA, ATRO''PIUM*, L. An alkaloid discovered by Brandes in _at'ropa belladon'na_ or deadly nightshade.

_Prep._ 1. (B. P. Process.) Take of belladonna-root, recently dried, and in coarse powder, 2 _lbs._; rectified spirit, 10 _pints_; slaked lime, 1 _oz._; diluted sulphuric acid, carbonate of potash, of each a sufficiency; chloroform, 3 _fl. oz._; purified animal charcoal, a sufficiency; distilled water, 10 _fl. oz._ Macerate the root in 4 pints of the spirit, for 24 hours, with frequent stirring. Transfer to a displacement apparatus, and exhaust the root with the remainder of the spirit by slow percolation. Add the lime to the tincture placed in a bottle, and shake them occasionally several times. Filter, add the diluted sulphuric acid in very feeble excess to the filtrate, and filter again. Distil off three fourths of the spirit, add to the residue the distilled water, evaporate at a gentle heat, but as rapidly as possible, until the liquor is reduced to one third of its volume and no longer smells of alcohol; then let it cool. Add very cautiously, with constant stirring, a solution of carbonate of potash so as nearly to neutralise the acid, care, however, being taken that an excess is not used. Set to rest for six hours, then filter, and add carbonate of potash in such quantity that the liquid shall acquire a decided alkaline reaction. Place in a bottle with the chloroform; mix well by frequently repeated brisk agitation, and pour the mixed liquids into a funnel furnished with a glass stop-cock. When the chloroform has subsided, draw it off by the stop-cock, and distil it on a water-bath from a retort connected with a condenser. Dissolve the residue in warm rectified spirit; digest the solution with a little animal charcoal: filter, evaporate, and cool until colourless crystals are obtained.

2. Expressed juice of belladonna is evaporated over a water-bath to the consistence of an extract, and then triturated in a marble or porcelain mortar with a strong solution of caustic potassa; the resulting mass is digested and well agitated for some time, at the temperature of 75° to 80° Fahr., with benzole, q. s.; and, after repose, the benzole-solution is carefully separated, and its volatile hydrocarbon is distilled off by the heat of a water-bath; the residuum in the retort is now exhausted with water acidulated with sulphuric acid, and the resulting 'acid-solution,' after filtration, precipitated with carbonate of soda; the precipitate is crude ATROPIA, which is collected on a filter, pressed between folds of bibulous paper, and dried; after which it is purified by one or more re-solutions, in alcohol, and crystallisations, which may or may not be modified in the manner noticed. The proportion of potassa should be about 1 dr. to every quart of the expressed juice. An excellent and economical process. The product is 0·3 to 4% of the weight of the plant from which the juice has been obtained.

3. (Mein and Liebig.) Belladonna-root (fresh-dried and coarsely powdered) is exhausted by alcohol (sp. gr. 0·822); slaked lime (1 part for every 24 of the dried root employed) is then added to the tincture, and the whole digested, with agitation, for 24 hours; sulphuric acid is next added, drop by drop, to slight excess, and, after filtration, rather more than one half the spirit is removed by distillation; a little water is now added to the residue, and the remainder of the alcohol evaporated as quickly as possible by a gentle heat; after again filtering, the liquid is reduced by further evaporation to the 1/12th part of the weight of the root employed, and a concentrated solution of potassa dropped into the cold liquid (to throw down a dark greyish-brown matter), carefully avoiding excess or rendering the liquid in the slightest degree alkaline; in a few hours the liquid is again filtered, and carbonate of potassa added as long as a precipitate (ATROPIA) falls; after a further interval of from 12 to 24 hours, this precipitate is collected and drained in a filter, and after pressure between folds of blotting-paper, dried by a very gentle heat. It is purified by making it into a paste with water, again squeezing it between the folds of blotting-paper, drying it, re-dissolving it in 5 times its weight of alcohol, decolouring it with pure animal charcoal, distilling off greater part of the alcohol, and evaporation and crystallisation by a very gentle heat; or only about one half the spirit is distilled off, and 3 or 4 times its volume of water gradually agitated with it, the resulting milky liquid being then heated to boiling, and allowed to cool very slowly, when nearly the whole of the ATROPIA crystallises out after a few hours. The same may be effected by at once agitating 6 or 8 volumes of water with the alcoholic solution, and setting aside the mixture for 12 to 24 hours, by which time the crystallisation will be completed. This process originated with Soubeiran, was improved by Mein, and subsequently, with slight modifications, adopted by Liebig. The product is about 0·3% of the weight of root operated on.

4. (Bouchardat and Cooper.) The filtered tincture is precipitated with iodine dissolved in an aqueous solution of iodide of potassium, the resulting ioduretted hydriodate of atropia, decomposed by zinc-and-water, the metallic oxide separated by means of carbonate of potassa, and the alkaloid thus obtained dissolved in alcohol and crystallised.

5. (Mr Luxton.) The dry leaves of belladonna are gently boiled for 2 hours in distilled water just sufficient to cover them, and the resulting decoction is strained through a coarse cloth into a large precipitating jar; this process is repeated with a second quantity of distilled water, and the two decoctions mixed; concentrated sulphuric acid is now added in the proportion of 2 dr. to every pound of leaves operated on, by which the vegetable albumen of the decoction is precipitated, and the liquid becomes clear and sherry-coloured; the clear liquor is now decanted or syphoned off, and, if necessary, filtered; the filtrate is now decomposed by either passing a stream of gaseous ammonia through it, or by suspending in it a lump of carbonate of ammonia. The effect is that the liquid turns black, and crystals of ATROPIA are slowly formed and deposited. At the expiration of a day or two, the supernatant mother-liquid is removed with a syphon, and the crystals thrown on a filter to drain and dry.[102] It may be purified by re-solution and crystallisation. 1 lb. of leaves yields 40 gr.; or at the rate of fully ·57%.

[Footnote 102: 'Pharm. Journ.,' 1854-5, p. 209{?}.]

6. (Rabourdin.) To the crystallised juice of the plant (previously heated to coagulate its albumen, filtered, and allowed to cool), 1 quart, is added of caustic potassa 1 dr., and afterwards of chloroform 1 oz.; the whole is then agitated well, and after half an hour's repose, the supernatant liquor is poured from the discoloured chloroform, which, after being washed with distilled water as long as it gives any colour to that liquid, is placed in a small retort, and the chloroform distilled off by the heat of a water-bath; the residuum is dissolved in a little water acidulated with sulphuric acid, and precipitated with carbonate of potassa, in slight excess; the precipitate is redissolved in alcohol, and the solution, by spontaneous evaporation, yields crystals of ATROPIA.

7. (Ure.) From the expressed juice of the fresh, or the watery extract of the dry plant, by treating it with caustic soda, in slight excess, and then agitating the mixture with 1-1/2 times its volume of ether; the ATROPIA taken up by the ether is again deposited after repose for some time, and is then purified by repeating the treatment with fresh ether as often as necessary.

8. Freshly precipitated hydrate of magnesia is added to the coagulated and filtered expressed juice, and the mixture evaporated to dryness, as quickly as possible, in a water-bath; the residuum is pulverised and digested in strong alcohol, and the clear liquid allowed to evaporate spontaneously. The crystals may be purified by repeated re-solutions in alcohol.

_Prop., Tests, &c._ The crystals obtained from hot concentrated solutions, colourless, transparent, silky prisms; from solutions in dilute spirit, silky needles, like those of disulphate of quinine. It is colourless; has a bitter, acrid, and somewhat metallic taste; dissolves in 200 parts (300 parts--Thomson) of cold and 50 to 54 parts of boiling water, in 1-1/2 parts of cold alcohol, and in 25 parts of cold, and 6 parts of boiling ether; it has an alkaline reaction, fuses at about 194° Fahr., is slightly volatile at common temperatures, and freely rises in vapour at 212° Fahr.; at higher temperatures it volatilises with partial decomposition; with the acids it forms salts, of which several are crystallisable.

_Tests._--1. Nitric acid forms with it a yellow solution:--2. With cold sulphuric acid it gives a colourless solution, which becomes red only when heated:--3. Aqueous solutions of atropia and its salts are--_a_, turned red by tincture of iodine--_b_, gives a citron-yellow precipitate with terchloride of gold--_c_, a flocculent whitish precipitate with tincture of galls, and--_d_, a yellowish-white one with bichloride of platinum:--4. Heated with caustic potassa or soda, it suffers decomposition, and ammonia is evolved:--5. A weak solution cautiously applied to the eyelid or conjunctiva, produces dilation of the pupil lasting for several hours.

_Pur., &c._ Alkaloid prepared from the root of atropa belladonna. Crystals; white, in the form of prisms; soluble in water and rectified spirit. It leaves no ash when burned with free access of air (B. P.).

_Phys. eff._ It is a very powerful narcotico-acrid poison.[103] Its effects are similar to those of belladonna, but considerably more powerful. "A very minute (imponderable) quantity applied to the eye is sufficient to dilate the pupil." (Pereira.) The 1/12 to 1/10 gr. often causes very serious effects in the human subject. The 1/6th of a grain accelerates the pulse, affects the brain, causes dryness of the throat, difficulty of deglutition, dilation of the pupil, dimness of sight, giddiness, strangury, numbness of limbs, sense of formication in the arms, rigidity of thighs, depression of pulse, and sometimes feebleness or loss of voice. These symptoms continue for from 12 to 24 hours. In larger doses death ensues.

[Footnote 103: A "cerebro-spinal poison."--Taylor.]

_Ant., &c._ These may be similar to those described under BELLADONNA and ALKALOID.

_Uses._ Chiefly as an external agent, as a substitute for belladonna, to cause dilation of the pupil; and as a local anæsthetic or anodyne, especially in facial neuralgia. Internally, it has been occasionally given in hooping-cough, chorea, and a few other nervous diseases.--_Dose_, 1/30 gr., gradually increased to 1/20, or, occasionally, even 1/15 gr. in solution, or made into a pill with liquorice powder and honey, or syrup, or used endermically; for a collyrium, 1 gr. to water 1 oz., a few drops only being applied to the eye at a time, the greatest caution in each case being observed. It is also employed to make the sulphate. In dispensing it a single drop of acetic acid, or dilute sulphuric acid, will be found to facilitate and ensure its perfect solution. See BELLADONNA and BELLADONINE.

=Atropia, Sul'phate of.= _Syn._ ATRO'PIA SUL'PHAS, L. _Prep._ (B. P.) Take of atropia, 120 _gr._; distilled water, 4 _fl. dr._; diluted sulphuric acid, a sufficiency.

Mix the atropia with the water and add the acid gradually, stirring them together until the alkaloid is dissolved and the solution is neutral. Evaporate it to dryness at a temperature not exceeding 100°.

_Characters and Tests._--A colourless powder, soluble in water, forming a solution which is neutral to test-paper, and when applied to the eye dilates the pupil as the solution of atropia does. It leaves no ash when burned with free access of air.

Intended for external application. It is a powerful poison.

_Uses, &c._ The same as those of the pure alkaloid.--_Dose_, 1/25 to 1/20 gr., either in solution or pills; 1 to 3 gr. to water 1 fl. oz., as a collyrium, of which a few drops seldom fail to produce full dilation of the pupil in about a quarter of an hour; 1 to 2 gr. to lard 1 dr. forms an excellent ointment in neuralgic affections.

_Obs._ Sulphate of atropia (which is intended for external use only) is rather difficult to crystallise, as it has a tendency to assume an amorphous or gum-like condition. It is more soluble than the pure alkaloid; and, like it, is a terrific poison.

=ATROPIA, VALERIANATÈ.= The Paris Codex directs this salt to be prepared as follows:--Dissolve valerianic acid in ether, and add atropia just sufficient to saturate the acid. Let the ether evaporate.

=ATROP'IC ACID.= _Syn._ ACIDUM ATROP'ICUM, L. The name given by Richter to a volatile crystallisable substance, possessing acid properties, found in atropa belladonna or deadly nightshade. In many respects it resembles benzoic acid, from which, however it is distinguished by not precipitating the salts of iron.

=ATROPI'NA, At'ropine.= See ATROPIA.

=AT'TAR.= See OTTO and VOLATILE OILS.

=ATTELETTES= (-l[)e]ts'). [Fr.] In _cookery_, small skewers, generally of silver, with ornamental heads. The term is also applied to small dishes (ENTRÉES, &c.) in which the articles are mounted on attelettes. Small fish, as smelts, are often served in this way. See AIGUILLETTE.

=ATTEN'UANT= (-[=u]-[)a]nt). _Syn._ ATTEN'UANS, L.; ATTÉNUANT, Fr.; VERDÜNNEND, Ger. That makes thin, or less dense or viscid; diluting. In _medicine_, applied to remedies (ATTEN'UANTS, SPANÆM'ICS) which are supposed to act by thinning, diluting, or impoverishing the blood.

=ATTENUA'TION.= _Syn._ ATTENUA'TIO, L.; ATTÉNUATION, Fr.; VERDÜNNUNG, Ger. A thinning or diminishing; a reducing in consistence. In _medicine_, see the adj. (_above_); in _brewing_, the decrease of the density of worts during fermentation, arising from the gradual conversion of their 'saccharine' (sugar) into alcohol. See BREWING, DISTILLATION, WORTS, &c.

=ATTRAC'TION.= [Eng., Fr.] _Syn._ ATTRAC'TIO, L.; ANZIEHUNG, Ger. The power that draws together matter and resists its separation. That force which attracts bodies towards the centre of the earth, and which keeps on its surface those that are movable, is called GRAVITY, or the attraction of gravitation. It is exerted at sensible, often at immense, distances, and determines the figure and motions of the planets and comets, and causes the descent of heavy bodies to the ground. This force it is which confers the property of weight upon matter.

That force which unites particles of the same kind of matter, so as to cause them to assume the condition of solid or liquid masses, _e.g._ particles of chalk to form a mass of chalk, particles of water to form a mass of water, is called COHESION, or the ATTRACTION OF COHESION. That force which binds together different substances without changing their properties, as when paint sticks to wood, ink to paper, &c., is called ADHESION, or the ATTRACTION OF ADHESION. CAPILLARY ATTRACTION is a modification of adhesion, and is characterised by being exerted between liquids and the internal surfaces of tubes and pervious bodies. The absorption of water by a sponge, the ascent of oil in the wick of a lamp, are examples of this power. The CHEMICAL FORCE or AFFINITY differs from all other kinds of attraction in being exerted between definite and constant quantities (atoms) of matter, usually of dissimilar natures, and producing combinations possessing properties different from those of their components. (See AFFINITY.) This force, as well as cohesion and adhesion, is exerted at distances so small as to be immeasurable.

The terms ELEC'TRIC ATTRACTION and MAGNET'IC ATTRACTION are employed in _physics_ to denote phenomena which we imperfectly understand, and which operate between bodies at sensible distances, and simulate those of the attraction of gravitation.

=ATTRI''TION= (tr[)i]sh'-[)u]n). [Eng., Fr.] _Syn._ ATTRI''TIO, L.; ABREIBUNG, AUFREIBUNG, Ger. In _mechanics_, the wearing away of parts by friction. In _medicine_, a graze, abrasion, or solution of continuity of the cuticle, or the act which causes it. In _surgery_, the crushing or tearing away of any exterior portion of the body by violence. See ABRASION, ANTI-ATTRITION, FRICTION, &c.

=AURANTIA'CEÆ= (-she-[=e]). [Lat.; DC.] The orange tribe. In _botany_, an extensive and important natural order of exogenous trees and shrubs, found exclusively in the temperate and tropical parts of the Old World, and unknown in a wild state in America. The fruit is pulpy, succulent, sub-acid, and eatable, and separated into cells by membranous partitions, and is covered with a leathery aromatic skin or rind. Some of the genera embrace plants of great beauty and utility. A few of the Indian species are climbers. The genus CIT'RUS, which includes the orange, lemon, citron, lime, bergamot, and shaddock, is that best known in Europe.

=AURAN''TIIN= (-she-[)i]n). _Syn._ HESPERIDIN; AURAN'TINE* (-t[)i]n), Eng., Fr.; AURANTII'NA, &c., L. The bitter principle of the peel of oranges and lemons.

_Prep._ The exterior or yellow peel of the Seville orange (carefully separated from the white matter, and air-dried) is steeped in hot water, and the filtered liquor gently evaporated to dryness.

_Prop._, _&c._ It possesses the bitter properties of the peel without any of its glutinosity or fragrance, and is said to agree better with delicate stomachs. It may be taken in water either with or without the addition of a little sugar or capillaire, or dissolved in wine.

=AU''RIC= (aw'- or awr'-). _Syn._ AURI'CUS, L. Of or relating to gold, or containing it, or formed from it.

=AURIF'EROUS.= _Syn._ AU''RIFER, AURIF'ERUS, L.; AURIFÈRE, Fr.; GOLDHALTIG, Ger. In, _mineralogy_, that yields or contains gold; as auriferous sand, a. quartz, &c.

=AURIPIGMEN'TUM[dagger].= [L.] Literally, paint of gold; appr., native orpiment. See ARSENIC.

=AURO-CHLO''RIDES= (kl[=o]re'-[=i]dz). Compounds of terchloride of gold with chlorides of other bases. They may be prepared by mixing the terchloride of gold with the chloride of the base, in atomic proportions, and setting aside the solution to crystallise.

_Prop._, _&c._ Most of the auro-chlorides crystallise in prisms, dissolve in both alcohol and water, have an orange or yellow colour, and are decomposed at a red heat.

=AURO-CY'ANIDES= ([)i]dz). In _chemistry_, compounds of cyanide of gold with cyanides of other bases. They may be formed in a similar manner to the auro-chlorides. Auro-cyanide of potassium is much used in electro-gilding.

=AURORA BOREALIS.= This luminous phenomenon, which is occasionally seen in our own country on clear frosty nights, and much more frequently and vividly by the dwellers in more northern latitudes, has been supposed to have an electrical origin, and to be occasioned by the passage of electricity through the rarefied strata of the upper regions of the atmosphere from the poles towards the equator. But physicists look upon this explanation as unsatisfactory, and inadequate to account for the effects produced. The hypothesis, however, seems to derive some support from the following fact:--

If one of Gassiot's vacuum tubes be brought near to a powerful electrical machine, both while the machine is in motion and for some time after, flashes of light may be seen passing from the wire at one end of the tube to the other extremity, which flashes bear a great resemblance to the auroral rays. The great doubt, however, is whether the conditions necessary to the production of the aurora are similar to those prevailing during this experiment, a doubt not lessened by the difficulty of satisfactorily accounting for the rarefied state of the atmosphere which is assumed to exist.

The forms which the aurora assumes are very varied and of great beauty; there appears, however, to be some general similarity in its aspect at the same locality. Its appearance is briefly as follows:--A dingy aspect in the heavens in a northernly direction is usually the precursor of the aurora; and this gradually becomes darker in colour, and assumes the form of a circular segment surrounded by a luminous arch, and resting at each end on the horizon. This dark segment presents the appearance of a thick cloud, and is frequently seen as such in the fading twilight, before the auroral light manifests itself. The density of this segment must, however, be very inconsiderable, as stars may sometimes be seen shining brightly through it.