Chapter XII: Preface (11)

Thus, you see, it is the most easy process imaginable to convert _nitrous oxyd gas_ into _nitrous acid gas_. The property of attracting oxygen from the atmosphere, without any elevation of temperature, has occasioned this gaseous oxyd being used as a test for ascertaining the degree of purity of the atmosphere. I am going to show you how it is applied to this purpose. --You see this graduated glass tube, which is closed at one end, (PLATE X. Fig. 2.) --I first fill it with water, and then introduce a certain measure of nitrous gas, which, not being absorbable by water, passes through it, and occupies the upper part of the tube. I must now add rather above two-thirds of oxygen gas, which will just be sufficient to convert the nitrous oxyd gas into nitrous acid gas.

CAROLINE.

So it has! --I saw it turn of an orange colour; but it immediately afterwards disappeared entirely, and the water, you see, has risen, and almost filled the tube.

MRS. B.

That is because the acid gas is absorbable by water, and in proportion as the gas impregnates the water, the latter rises in the tube. When the oxygen gas is very pure, and the required proportion of nitrous oxyd gas very exact, the whole is absorbed by the water; but if any other gas be mixed with the oxygen, instead of combining with the nitrous oxygen, it will remain and occupy the upper part of the tube; or, if the gases be not in the due proportion, there will be a residue of that which predominates. --Before we leave this subject, I must not forget to remark that nitrous acid may be formed by dissolving nitrous oxyd gas in nitric acid. This solution may be effected simply by making bubbles of nitrous oxyd gas pass through nitric acid.

EMILY.

That is to say, that nitrogen at its highest degree of oxygenation, being mixed with nitrogen at its lowest degree of oxygenation, will produce a kind of intermediate substance, which is nitrous acid.

MRS. B.

You have stated the fact with great precision. --There are various other methods of preparing nitrous oxyd, and of obtaining it from compound bodies; but it is not necessary to enter into these particulars. It remains for me only to mention another curious modification of oxygenated nitrogen, which has been distinguished by the name of _gaseous oxyd of nitrogen_. It is but lately that this gas has been accurately examined, and its properties have been investigated chiefly by Sir H. Davy. It has obtained also the name of _exhilarating_ gas, from the very singular property which that gentleman has discovered in it, of elevating the animal spirits, when inhaled into the lungs, to a degree sometimes resembling delirium or intoxication.

CAROLINE.

Is it respirable, then?

MRS. B.

It can scarcely be called respirable, as it would not support life for any length of time; but it may be breathed for a few moments without any other effects, than the singular exhilaration of spirits I have just mentioned. It affects different people, however, in a very different manner. Some become violent, even outrageous: others experience a languor, attended with faintness; but most agree in opinion, that the sensations it excites are extremely pleasant.

CAROLINE.

I think I should like to try it--how do you breathe it?

MRS. B.

By collecting the gas in a bladder, to which a short tube with a stop-cock is adapted; this is applied to the mouth with one hand, whilst the nostrils are kept closed with the other, that the common air may have no access. You then alternately inspire, and expire the gas, till you perceive its effects. But I cannot consent to your making the experiment; for the nerves are sometimes unpleasantly affected by it, and I would not run any risk of that kind.

EMILY.

I should like, at least, to see somebody breathe it; but pray by what means is this curious gas obtained?

MRS. B.

It is procured from _nitrat of ammonia_, an artificial salt which yields this gas on the application of a gentle heat. I have put some of the salt into a retort, and by the aid of a lamp the gas will be extricated.--

CAROLINE.

Bubbles of air begin to escape through the neck of the retort into the water apparatus; will you not collect them?

MRS. B.

The gas that first comes over need not be preserved, as it consists of little more than the common air that was in the retort; besides, there is always in this experiment a quantity of watery vapour which must come away before the nitrous oxyd appears.

EMILY.

Watery vapour! Whence does that proceed? There is no water in nitrat of ammonia?

MRS. B.

You must recollect that there is in every salt a quantity of water of crystallisation, which may be evaporated by heat alone. But, besides this, water is actually generated in this experiment, as you will see presently. First tell me, what are the constituent parts of nitrat of ammonia?

EMILY.

Ammonia, and nitric acid: this salt, therefore, contains three different elements, nitrogen and hydrogen, which produce the ammonia; and oxygen, which, with nitrogen, forms the acid.

MRS. B.

Well then, in this process the ammonia is decomposed; the hydrogen quits the nitrogen to combine with some of the oxygen of the nitric acid, and forms with it the watery vapour which is now coming over. When that is effected, what will you expect to find?

EMILY.

Nitrous acid instead of nitric acid, and nitrogen instead of ammonia.

MRS. B.

Exactly so; and the nitrous acid and nitrogen combine, and form the gaseous oxyd of nitrogen, in which the proportion of oxygen is 37 parts to 63 of nitrogen.

You may have observed, that for a little while no bubbles of air have come over, and we have perceived only a stream of vapour condensing as it issued into the water. --Now bubbles of air again make their appearance, and I imagine that by this time all the watery vapour is come away, and that we may begin to collect the gas. We may try whether it is pure, by filling a phial with it, and plunging a taper into it--yes, it will do now, for the taper burns brighter than in the common air, and with a greenish flame.

CAROLINE.

But how is that? I thought no gas would support combustion but oxygen or chlorine.

MRS. B.

Or any gas that contains oxygen, and is ready to yield it, which is the case with this in a considerable degree; it is not, therefore, surprising that it should accelerate the combustion of the taper.

You see that the gas is now produced in great abundance; we shall collect a large quantity of it, and I dare say that we shall find some of the family who will be curious to make the experiment of respiring it. Whilst this process is going on, we may take a general survey of the most important combinations of the nitric and nitrous acids with the alkalies.

The first of these is _nitrat of potash_, commonly called _nitre_ or _saltpetre_.

CAROLINE.

Is not that the salt with which gunpowder is made?

MRS. B.

Yes. Gunpowder is a mixture of five parts of nitre to one of sulphur, and one of charcoal. --Nitre from its great proportion of oxygen, and from the facility with which it yields it, is the basis of most detonating compositions.

EMILY.

But what is the cause of the violent detonation of gunpowder when set fire to?

MRS. B.

Detonation may proceed from two causes; the sudden formation or destruction of an elastic fluid. In the first case, when either a solid or liquid is instantaneously converted into an elastic fluid, the prodigious and sudden expansion of the body strikes the air with great violence, and this concussion produces the sound called detonation.

CAROLINE.

That I comprehend very well; but how can a similar effect be produced by the destruction of a gas?

MRS. B.

A gas can be destroyed only by condensing it to a liquid or solid state; when this takes place suddenly, the gas, in assuming a new and more compact form, produces a vacuum, into which the surrounding air rushes with great impetuosity; and it is by that rapid and violent motion that the sound is produced. In all detonations, therefore, gases are either suddenly formed, or destroyed. In that of gunpowder, can you tell me which of these two circumstances takes place?

EMILY.

As gunpowder is a solid, it must, of course, produce the gases in its detonation; but how, I cannot tell.

MRS. B.

The constituents of gunpowder, when heated to a certain degree, enter into a number of new combinations, and are instantaneously converted into a variety of gases, the sudden expansion of which gives rise to the detonation.

CAROLINE.

And in what instance does the destruction or condensation of gases produce detonation?

MRS. B.

I can give you one with which you are well acquainted; the sudden combination of the oxygen and hydrogen gases.

CAROLINE.

True; I recollect perfectly that hydrogen detonates with oxygen when the two gases are converted into water.

MRS. B.

But let us return to the nitrat of potash. --This salt is decomposed when exposed to heat, and mixed with any combustible body, such as carbon, sulphur, or metals, these substances oxydating rapidly at the expense of the nitrat. I must show you an instance of this. --I expose to the fire some of the salt in a small iron ladle, and, when it is sufficiently heated, add to it some powdered charcoal; this will attract the oxygen from the salt, and be converted into carbonic acid.--

EMILY.

But what occasions that crackling noise, and those vivid flashes that accompany it?

MRS. B.

The rapidity with which the carbonic acid gas is formed occasions a succession of small detonations, which, together with the emission of flame, is called _deflagration_.

_Nitrat of ammonia_ we have already noticed, on account of the gaseous oxyd of nitrogen which is obtained from it.

_Nitrat of silver_ is the lunar caustic, so remarkable for its property of destroying animal fibre, for which purpose it is often used by surgeons. --We have said so much on a former occasion, on the mode in which caustics act on animal matter, that I shall not detain you any longer on this subject.

We now come to the CARBONIC ACID, which we have already had many opportunities of noticing. You recollect that this acid may be formed by the combustion of carbon, whether in its imperfect state of charcoal, or in its purest form of diamond. And it is not necessary, for this purpose, to burn the carbon in oxygen gas, as we did in the preceding lecture; for you need only light a piece of charcoal and suspend it under a receiver on the water bath. The charcoal will soon be extinguished, and the air in the receiver will be found mixed with carbonic acid. The process, however, is much more expeditious if the combustion be performed in pure oxygen gas.

CAROLINE.

But how can you separate the carbonic acid, obtained in this manner, from the air with which it is mixed?

MRS. B.

The readiest mode is to introduce under the receiver a quantity of caustic lime, or caustic alkali, which soon attracts the whole of the carbonic acid to form a carbonat. --The alkali is found increased in weight, and the volume of the air is diminished by a quantity equal to that of the carbonic acid which was mixed with it.

EMILY.

Pray is there no method of obtaining pure carbon from carbonic acid?

MRS. B.

For a long time it was supposed that carbonic acid was not decompoundable; but Mr. Tennant discovered, a few years ago, that this acid may be decomposed by burning phosphorus in a closed vessel with carbonat of soda or carbonat of lime: the phosphorus absorbs the oxygen from the carbonat, whilst the carbon is separated in the form of a black powder. This decomposition, however, is not effected simply by the attraction of the phosphorus for oxygen, since it is weaker than that of charcoal; but the attraction of the alkali of lime for the phosphoric acid, unites its power at the same time.

CAROLINE.

Cannot we make that experiment?

MRS. B.

Not easily; it requires being performed with extreme nicety, in order to obtain any sensible quantity of carbon, and the experiment is much too delicate for me to attempt it. But there can be no doubt of the accuracy of Mr. Tennant’s results; and all chemists now agree, that one hundred parts of carbonic acid gas consists of about twenty-eight parts of carbon to seventy-two of oxygen gas. But if you recollect, we decomposed carbonic acid gas the other day by burning potassium in it.

CAROLINE.

True, so we did; and found the carbon precipitated on the regenerated potash.

MRS. B.

Carbonic acid gas is found very abundantly in nature; it is supposed to form about one thousandth part of the atmosphere, and is constantly produced by the respiration of animals; it exists in a great variety of combinations, and is exhaled from many natural decompositions. It is contained in a state of great purity in certain caves, such as the _Grotto del Cane_, near Naples.

EMILY.

I recollect having read an account of that grotto, and of the cruel experiments made on the poor dogs, to gratify the curiosity of strangers. But I understood that the vapour exhaled by this cave was called _fixed air_.

MRS. B.

That is the name by which carbonic acid was known before its chemical composition was discovered. --This gas is more destructive of life than any other; and if the poor animals that are submitted to its effects are not plunged into cold water as soon as they become senseless, they do not recover. It extinguishes flame instantaneously. I have collected some in this glass, which I will pour over the candle.

CAROLINE.

This is extremely singular--it seems to extinguish it as it were by enchantment, as the gas is invisible. I never should have imagined that gas could have been poured like a liquid.

MRS. B.

It can be done with carbonic acid only, as no other gas is sufficiently heavy to be susceptible of being poured out in the atmospherical air without mixing with it.

EMILY.

Pray by what means did you obtain this gas?

MRS. B.

I procured it from marble. Carbonic acid gas has so strong an attraction for all the alkalies and alkaline earths, that these are always found in nature in the state of carbonats. Combined with lime, this acid forms chalk, which may be considered as the basis of all kinds of marbles, and calcareous stones. From these substances carbonic acid is easily separated, as it adheres so slightly to its combinations, that the carbonats are all decomposable by any of the other acids. I can easily show you how I obtained this gas; I poured some diluted sulphuric acid over pulverised marble in this bottle (the same which we used the other day to prepare hydrogen gas), and the gas escaped through the tube connected with it; the operation still continues, as you may easily perceive--

EMILY.

Yes, it does; there is a great fermentation in the glass vessel. What singular commotion is excited by the sulphuric acid taking possession of the lime, and driving out the carbonic acid!

CAROLINE.

But did the carbonic acid exist in a gaseous state in the marble?

MRS. B.

Certainly not; the acid, when in a state of combination, is capable of existing in a solid form.

CAROLINE.

Whence, then, does it obtain the caloric necessary to convert it into gas?

MRS. B.

It may be supplied in this case from the mixture of sulphuric acid and water, which produces an evolution of heat, even greater than is required for the purpose; since, as you may perceive by touching the glass vessel, a considerable quantity of the caloric disengaged becomes sensible. But a supply of caloric may be obtained also from a diminution of capacity for heat, occasioned by the new combination which takes place; and, indeed, this must be the case when other acids are employed for the disengagement of carbonic acid gas, which do not, like the sulphuric, produce heat on being mixed with water. Carbonic acid may likewise be disengaged from its combinations by heat alone, which restores it to its gaseous state.

CAROLINE.

It appears to me very extraordinary that the same gas, which is produced by the burning of wood and coals, should exist also in such bodies as marble, and chalk, which are incombustible substances.

MRS. B.

I will not answer that objection, Caroline, because I think I can put you in a way of doing it yourself. Is carbonic acid combustible?

CAROLINE.

Why, no--because it is a body that has been already burnt; it is carbon only, and not the acid, that is combustible.

MRS. B.

Well, and what inference do you draw from this?

CAROLINE.

That carbonic acid cannot render the bodies with which it is united combustible; but that simple carbon does, and that it is in this elementary state that it exists in wood, coals, and a great variety of other combustible bodies. --Indeed, Mrs. B., you are very ungenerous; you are not satisfied with convincing me that my objections are frivolous, but you oblige me to prove them so myself.

MRS. B.

You must confess, however, that I make ample amends for the detection of error, when I enable you to discover the truth. You, understand, now, I hope, that carbonic acid is equally produced by the decomposition of chalk, or by the combustion of charcoal. These processes are certainly of a very different nature; in the first case the acid is already formed, and requires nothing more than heat to restore it to its gaseous state; whilst, in the latter, the acid is actually made by the process of combustion.

CAROLINE.

I understand it now perfectly. But I have just been thinking of another difficulty, which, I hope, you will excuse my not being able to remove myself. How does the immense quantity of calcareous earth, which is spread all over the globe, obtain the carbonic acid with which it is combined?

MRS. B.

The question is, indeed, not very easy to answer; but I conceive that the general carbonisation of calcareous matter may have been the effect of a general combustion, occasioned by some revolution of our globe, and producing an immense supply of carbonic acid, with which the calcareous matter became impregnated; or that this may have been effected by a gradual absorption of carbonic acid from the atmosphere. --But this would lead us to discussions which we cannot indulge in, without deviating too much from our subject.

EMILY.

How does it happen that we do not perceive the pernicious effects of the carbonic acid which is floating in the atmosphere?

MRS. B.

Because of the state of very great dilution in which it exists there. But can you tell me, Emily, what are the sources which keep the atmosphere constantly supplied with this acid?

EMILY.

I suppose the combustion of wood, coals, and other substances, that contain carbon.

MRS. B.

And also the breath of animals.

CAROLINE.

The breath of animals! I thought you said that this gas was not at all respirable, but on the contrary, extremely poisonous.

MRS. B.

So it is; but although animals cannot breathe in carbonic acid gas, yet, in the process of respiration, they have the power of forming this gas in their lungs; so that the air which we _expire_, or reject from the lungs, always contains a certain proportion of carbonic acid, which is much greater than that which is commonly found in the atmosphere.

CAROLINE.

But what is it that renders carbonic acid such a deadly poison?

MRS. B.

The manner in which this gas destroys life, seems to be merely by preventing the access of respirable air; for carbonic acid gas, unless very much diluted with common air, does not penetrate into the lungs, as the windpipe actually contracts and refuses it admittance. --But we must dismiss this subject at present, as we shall have an opportunity of treating of respiration much more fully, when we come to the chemical functions of animals.

EMILY.

Is carbonic acid as destructive to the life of vegetables as it is to that of animals?

MRS. B.

If a vegetable be completely immersed in it, I believe it generally proves fatal to it; but mixed in certain proportions with atmospherical air, it is, on the contrary, very favourable to vegetation.

You remember, I suppose, our mentioning the mineral waters, both natural and artificial, which contain carbonic acid gas?

CAROLINE.

You mean the Seltzer water?

MRS. B.

That is one of those which are the most used; there are, however, a variety of others into which carbonic acid enters as an ingredient: all these waters are usually distinguished by the name of _acidulous_ or _gaseous mineral waters_.

The class of salts called _carbonats_ is the most numerous in nature; we must pass over them in a very cursory manner, as the subject is far too extensive for us to enter on it in detail. The state of carbonat is the natural state of a vast number of minerals, and particularly of the alkalies and alkaline earths, as they have so great an attraction for the carbonic acid, that they are almost always found combined with it; and you may recollect that it is only by separating them from this acid, that they acquire that causticity and those striking qualities which I have formerly described. All marbles, chalks, shells, calcareous spars, and lime-stones of every description, are neutral salts, in which _lime_, their common basis, has lost all its characteristic properties.

EMILY.

But if all these various substances are formed by the union of lime with carbonic acid, whence arises their diversity of form and appearance?

MRS. B.

Both from the different proportions of their component parts, and from a variety of foreign ingredients which may be occasionally blended with them: the veins and colours of marbles, for instance, proceed from a mixture of metallic substances; silex and alumine also frequently enter into these combinations. The various carbonats, therefore, that I have enumerated, cannot be considered as pure unadulterated neutral salts, although they certainly belong to that class of bodies.

CONVERSATION XIX.

ON THE BORACIC, FLUORIC, MURIATIC, AND OXYGENATED MURIATIC ACIDS; AND ON MURIATS. --ON IODINE AND IODIC ACID.

MRS. B.

We now come to the three remaining acids with simple bases, the compound nature of which, though long suspected, has been but recently proved. The chief of these is the muriatic; but I shall first describe the two others, as their bases have been obtained more distinctly than that of the muriatic acid.

You may recollect I mentioned the BORACIC ACID. This is found very sparingly in some parts of Europe, but for the use of manufactures we have always received it from the remote country of Thibet, where it is found in some lakes, combined with soda. It is easily separated from the soda by sulphuric acid, and appears in the form of shining scales, as you see here.

CAROLINE.

I am glad to meet with an acid which we need not be afraid to touch; for I perceive, from your keeping it in a piece of paper, that it is more innocent than our late acquaintance, the sulphuric and nitric acids.

MRS. B.

Certainly; but being more inert, you will not find its properties so interesting. However, its decomposition, and the brilliant spectacle it affords when its basis again unites with oxygen, atones for its want of other striking qualities.

Sir H. Davy succeeded in decomposing the boracic acid, (which had till then been considered as undecompoundable,) by various methods. On exposing this acid to the Voltaic battery, the positive wire gave out oxygen, and on the negative wire was deposited a black substance, in appearance resembling charcoal. This was the basis of the acid, which Sir H. Davy has called _Boracium_, or _Boron_.

The same substance was obtained in more considerable quantities, by exposing the acid to a great heat in an iron gun-barrel.

A third method of decomposing the boracic acid consisted in burning potassium in contact with it in vacuo. The potassium attracts the oxygen from the acid, and leaves its basis in a separate state.

The recomposition of this acid I shall show you, by burning some of its basis, which you see here, in a retort full of oxygen gas. The heat of a candle is all that is required for this combustion.--

EMILY.

The light is astonishingly brilliant, and what beautiful sparks it throws out!

MRS. B.

The result of this combustion is the boracic acid, the nature of which, you see, is proved both by analytic and synthetic means. Its basis has not, it is true, a metallic appearance; but it makes very hard alloys with other metals.

EMILY.

But pray, Mrs. B., for what purpose is the boracic acid used in manufactures?

MRS. B.

Its principal use is in conjunction with soda, that is, in the state of _borat of soda_, which in the arts is commonly called borax. This salt has a peculiar power of dissolving metallic oxyds, and of promoting the fusion of substances capable of being melted; it is accordingly employed in various metallic arts; it is used, for example, to remove the oxyd from the surface of metals, and is often employed in the assaying of metallic ores.

Let us now proceed to the FLUORIC ACID. This acid is obtained from a substance which is found frequently in mines, and particularly in those of Derbyshire, called _fluor_, a name which it acquired from the circumstance of its being used to render the ores of metals more fluid when heated.

CAROLINE.

Pray is not this the Derbyshire spar, of which so many ornaments are made?

MRS. B.

The same; but though it has long been employed for a variety of purposes, its nature was unknown until Scheele, the great Swedish chemist, discovered that it consisted of lime united with a peculiar acid, which obtained the name of _fluoric acid_. It is easily separated from the lime by the sulphuric acid, and unless condensed in water, ascends in the form of gas. A very peculiar property of this acid is its union with siliceous earths, which I have already mentioned. If the distillation of this acid is performed in glass vessels, they are corroded, and the siliceous part of the glass comes over, united with the gas; if water is then admitted, part of the silex is deposited, as you may observe in this jar.

CAROLINE.

I see white flakes forming on the surface of the water; is that silex?

MRS. B.

Yes it is. This power of corroding glass has been used for engraving, or rather etching, upon it. The glass is first covered with a coat of wax, through which the figures to be engraved are to be scratched with a pin; then pouring the fluoric acid over the wax, it corrodes the glass where the scratches have been made.

CAROLINE.

I should like to have a bottle of this acid, to make engravings.

MRS. B.

But you could not have it in a _glass_ bottle, for in that case the acid would be saturated with silex, and incapable of executing an engraving; the same thing would happen were the acid kept in vessels of porcelain or earthen-ware; this acid must therefore be both prepared and preserved in vessels of silver.

If it be distilled from fluor spar and vitriolic acid, in silver or leaden vessels, the receiver being kept very cold during the distillation, it assumes the form of a dense fluid, and in that state is the most intensely corrosive substance known. This seems to be the acid combined with a little water. It may be called _hydro-fluoric acid_; and Sir H. Davy has been led, from some late experiments on the subject, to consider _pure_ fluoric acid as a compound of a certain unknown principle, which he calls _fluorine_, with hydrogen.

Sir H. Davy has also attempted to decompose the fluoric acid by burning potassium in contact with it; but he has not yet been able by this or any other method, to obtain its basis in a distinct separate state.

We shall conclude our account of the acids with that of the MURIATIC ACID, which is perhaps the most curious and interesting of all of them. It is found in nature combined with soda, lime, and magnesia. _Muriat of soda_ is the common sea-salt, and from this substance the acid is usually disengaged by means of the sulphuric acid. The natural state of the muriatic acid is that of an invisible permanent gas, at the common temperature of the atmosphere; but it has a remarkably strong attraction for water, and assumes the form of a whitish cloud whenever it meets any moisture to combine with. This acid is remarkable for its peculiar and very pungent smell, and possesses, in a powerful degree, most of the acid properties. Here is a bottle containing muriatic acid in a liquid state.

CAROLINE.

And how is it liquefied?

MRS. B.

By impregnating water with it; its strong attraction for water makes it very easy to obtain it in a liquid form. Now, if I open the phial, you may observe a kind of vapour rising from it, which is muriatic acid gas, of itself invisible, but made apparent by combining with the moisture of the atmosphere.

EMILY.

Have you not any of the pure muriatic acid gas?

MRS. B.

This jar is full of that acid in its gaseous state--it is inverted over mercury instead of water, because, being absorbable by water, this gas cannot be confined by it. --I shall now raise the jar a little on one side, and suffer some of the gas to escape. --You see that it immediately becomes visible in the form of a cloud.

EMILY.

It must be, no doubt, from its uniting with the moisture of the atmosphere, that it is converted into this dewy vapour.

MRS. B.

Certainly; and for the same reason, that is to say, its extreme eagerness to unite with water, this gas will cause snow to melt as rapidly as an intense fire.

This acid proved much more refractory when Sir H. Davy attempted to decompose it than the other two undecompounded acids. It is singular that potassium will burn in muriatic acid, and be converted into potash, without decomposing the acid, and the result of this combustion is a _muriat of potash_; for the potash, as soon as it is regenerated, combines with the muriatic acid.

CAROLINE.

But how can the potash be regenerated if the muriatic acid does not oxydate the potassium?

MRS. B.

The potassium, in this process, obtains oxygen from the moisture with which the muriatic acid is always combined, and accordingly hydrogen, resulting from the decomposition of the moisture, is invariably evolved.

EMILY.

But why not make these experiments with dry muriatic acid?

MRS. B.

Dry acids cannot be acted on by the Voltaic battery, because acids are non-conductors of electricity, unless moistened. In the course of a number of experiments which Sir H. Davy made upon acids in a state of dryness, he observed that the presence of water appeared always necessary to develop the acid properties, so that acids are not even capable of reddening vegetable blues if they have been carefully deprived of moisture. This remarkable circumstance led him to suspect, that water, instead of oxygen, may be the acidifying principle; but this he threw out rather as a conjecture than as an established point.

Sir H. Davy obtained very curious results from burning potassium in a mixture of phosphorus and muriatic acid, and also of sulphur and muriatic acid; the latter detonates with great violence. All his experiments, however, failed in presenting to his view the basis of the muriatic acid, of which he was in search; and he was at last induced to form an opinion respecting the nature of this acid, which I shall presently explain.

EMILY.

Is this acid susceptible of different degrees of oxygenation?

MRS. B.

Yes, for though we cannot deoxygenate this acid, yet we may add oxygen to it.

CAROLINE.

Why, then, is not the least degree of oxygenation of the acid called the _muriatous_, and the higher degree the _muriatic_ acid?

MRS. B.

Because, instead of becoming, like other acids, more dense, and more acid by an addition of oxygen, it is rendered on the contrary more volatile, more pungent, but less acid, and less absorbable by water. These circumstances, therefore, seem to indicate the propriety of making an exception to the nomenclature. The highest degree of oxygenation of this acid has been distinguished by the additional epithet of _oxygenated_, or, for the sake of brevity, _oxy_, so that it is called the _oxygenated_, or _oxy-muriatic acid_. This likewise exists in a gaseous form, at the temperature of the atmosphere; it is also susceptible of being absorbed by water, and can be congealed, or solidified, by a certain degree of cold.

EMILY.

And how do you obtain the oxy-muriatic acid?

MRS. B.

In various ways; but it may be most conveniently obtained by distilling liquid muriatic acid over oxyd of manganese, which supplies the acid with the additional oxygen. One part of the acid being put into a retort, with two parts of the oxyd of manganese, and the heat of a lamp applied, the gas is soon disengaged, and may be received over water, as it is but sparingly absorbed by it. --I have collected some in this jar--

CAROLINE.

It is not invisible, like the generality of gases; for it is of a yellowish colour.

MRS. B.

The muriatic acid extinguishes flame, whilst, on the contrary, the oxy-muriatic makes the flame larger, and gives it a dark red colour. Can you account for this difference in the two acids?

EMILY.

Yes, I think so; the muriatic acid will not supply the flame with the oxygen necessary for its support; but when this acid is further oxygenated, it will part with its additional quantity of oxygen, and in this way support combustion.

MRS. B.

That is exactly the case; indeed the oxygen added to the muriatic acid, adheres so slightly to it, that it is separated by mere exposure to the sun’s rays. This acid is decomposed also by combustible bodies, many of which it burns, and actually inflames, without any previous increase of temperature.

CAROLINE.

That is extraordinary, indeed! I hope you mean to indulge us with some of these experiments?

MRS. B.

I have prepared several glass jars of oxy-muriatic acid gas for that purpose. In the first we shall introduce some Dutch gold leaf. --Do you observe that it takes fire?

EMILY.

Yes, indeed it does--how wonderful it is! It became immediately red hot, but was soon smothered in a thick vapour.

CAROLINE.

What a disagreeable smell!

MRS. B.

We shall try the same experiment with phosphorus in another jar of this acid. --You had better keep your handkerchief to your nose when I open it--now let us drop into it this little piece of phosphorus--

CAROLINE.

It burns really; and almost as brilliantly as in oxygen gas! But, what is most extraordinary, these combustions take place without the metal or phosphorus being previously lighted, or even in the least heated.

MRS. B.

All these curious effects are owing to the very great facility with which this acid yields oxygen to such bodies as are strongly disposed to combine with it. It appears extraordinary indeed to see bodies, and metals in particular, melted down and inflamed, by a gas without any increase of temperature, either of the gas, or of the combustible. The phenomenon, however, is, you see, well accounted for.

EMILY.

Why did you burn a piece of Dutch gold leaf rather than a piece of any other metal?

MRS. B.

Because, in the first place, it is a composition of metals (consisting chiefly of copper) which burns readily; and I use a thin metallic leaf in preference to a lump of metal, because it offers to the action of the gas but a small quantity of matter under a large surface. Filings, or shavings, would answer the purpose nearly as well; but a lump of metal, though the surface would oxydate with great rapidity, would not take fire. Pure gold is not inflamed by oxy-muriatic acid gas, but it is rapidly oxydated, and dissolved by it; indeed, this acid is the only one that will dissolve gold.

EMILY.

This, I suppose, is what is commonly called _aqua regia_, which you know is the only thing that will act upon gold.

MRS. B.

That is not exactly the case either; for aqua regia is composed of a mixture of muriatic acid and nitric acid. --But, in fact, the result of this mixture is the formation of oxy-muriatic acid, as the muriatic acid oxygenates itself at the expence of the nitric; this mixture, therefore, though it bears the name of _nitro-muriatic acid_, acts on gold merely in virtue of the oxy-muriatic acid which it contains.

Sulphur, volatile oils, and many other substances, will burn in the same manner in oxy-muriatic acid gas; but I have not prepared a sufficient quantity of it, to show you the combustion of all these bodies.

CAROLINE.

There are several jars of the gas yet remaining.

MRS. B.

We must reserve these for future experiments. The oxy-muriatic acid does not, like other acids, redden the blue vegetable colours; but it totally destroys any colour, and turns all vegetables perfectly white. Let us collect some vegetable substances to put into this glass, which is full of gas.

EMILY.

Here is a sprig of myrtle--

CAROLINE.

And here some coloured paper--

MRS. B.

We shall also put in this piece of scarlet riband, and a rose--

EMILY.

Their colours begin to fade immediately! But how does the gas produce this effect?

MRS. B.

The oxygen combines with the colouring matter of these substances, and destroys it; that is to say, destroys the property which these colours had of reflecting only one kind of rays, and renders them capable of reflecting them all, which, you know, will make them appear white. Old prints may be cleaned by this acid, for the paper will be whitened without injury to the impression, as printer’s ink is made of materials (oil and lamp black) which are not acted upon by acids.

This property of the oxy-muriatic acid has lately been employed in manufactures in a variety of bleaching processes; but for these purposes the gas must be dissolved in water, as the acid is thus rendered much milder and less powerful in its effects; for, in a gaseous state, it would destroy the texture, as well as the colour of the substance submitted to its action.

CAROLINE.

Look at the things which we put into the gas; they have now entirely lost their colour!

MRS. B.

The effect of the acid is almost completed; and, if we were to examine the quantity that remains, we should find it to consist chiefly of muriatic acid.

The oxy-muriatic acid has been used to purify the air in fever hospitals and prisons, as it burns and destroys putrid effluvia of every kind. The infection of the small-pox is likewise destroyed by this gas, and matter that has been submitted to its influence will no longer generate that disorder.

CAROLINE.

Indeed, I think the remedy must be nearly as bad as the disease; the oxy-muriatic acid has such a dreadfully suffocating smell.

MRS. B.

It is certainly extremely offensive; but by keeping the mouth shut, and wetting the nostrils with liquid ammonia, in order to neutralize the vapour as it reaches the nose, its prejudicial effects may be in some degree prevented. At any rate, however, this mode of disinfection can hardly be used in places that are inhabited. And as the vapour of nitric acid, which is scarcely less efficacious for this purpose, is not at all prejudicial, it is usually preferred on such occasions.

CAROLINE.

You have not told us yet what is Sir H. Davy’s new opinion respecting the nature of muriatic acid, to which you alluded a few minutes ago?

MRS. B.

True; I avoided noticing it then, because you could not have understood it without some previous knowledge of the oxy-muriatic acid, which I have but just introduced to your acquaintance.

Sir H. Davy’s idea is that muriatic acid, instead of being a compound, consisting of an unknown basis and oxygen, is formed by the union of oxy-muriatic gas with hydrogen.

EMILY.

Have you not told us just now that oxy-muriatic gas was itself a compound of muriatic acid and oxygen?

MRS. B.

Yes; but according to Sir H. Davy’s hypothesis, oxy-muriatic gas is considered as a simple body, which contains no oxygen--as a substance of its own kind, which has a great analogy to oxygen in most of its properties, though in others it differs entirely from it. --According to this view of the subject, the name of _oxy-muriatic acid_ can no longer be proper, and therefore Sir H. Davy has adopted that of _chlorine_, or _chlorine gas_, a name which is simply expressive of its greenish colour; and in compliance with that philosopher’s theory, we have placed chlorine in our table among the simple bodies.

CAROLINE.

But what was Sir H. Davy’s reason for adopting an opinion so contrary to that which had hitherto prevailed?

MRS. B.

There are many circumstances which are favourable to the new doctrine; but the clearest and simplest fact in its support is, that if hydrogen gas and oxy-muriatic gas be mixed together, both these gases disappear, and muriatic acid gas is formed.

EMILY.

That seems to be a complete proof; is it not considered as perfectly conclusive?

MRS. B.

Not so decisive as it appears at first sight; because it is argued by those who still incline to the old doctrine, that muriatic acid gas, however dry it may be, always contains a certain quantity of water, which is supposed essential to its formation. So that, in the experiment just mentioned, this water is supplied by the union of the hydrogen gas with the oxygen of the oxy-muriatic acid; and therefore the mixture resolves itself into the base of muriatic acid and water, that is, muriatic acid gas.

CAROLINE.

I think the old theory must be the true one; for otherwise how could you explain the formation of oxy-muriatic gas, from a mixture of muriatic acid and oxyd of manganese?

MRS. B.

Very easily; you need only suppose that in this process the muriatic acid is decomposed; its hydrogen unites with the oxygen of the manganese to form water, and the chlorine appears in its separate state.

EMILY.

But how can you explain the various combustions which take place in oxy-muriatic gas, if you consider it as containing no oxygen?

MRS. B.

We need only suppose that combustion is the result of intense chemical action; so that chlorine, like oxygen, in combining with bodies, forms compounds which have less capacity for caloric than their constituent principles, and, therefore, caloric is evolved at the moment of their combination.

EMILY.

If, then, we may explain every thing by either theory, to which of the two shall we give the preference?

MRS. B.

It will, perhaps, be better to wait for more positive proofs, if such can be obtained, before we decide positively upon the subject. The new doctrine has certainly gained ground very rapidly, and may be considered as nearly established; but several competent judges still refuse their assent to it, and until that theory is very generally adopted, it may be as well for us still occasionally to use the language to which chemists have long been accustomed. --But let us proceed to the examination of salts formed by muriatic acid.

Among the compound salts formed by muriatic acid, the _muriat of soda_, or common salt, is the most interesting.* The uses and properties of this salt are too well known to require much comment. Besides the pleasant flavour it imparts to the food, it is very wholesome, when not used to excess, as it assists the process of digestion.

Sea-water is the great source from which muriat of soda is extracted by evaporation. But it is also found in large solid masses in the bowels of the earth, in England, and in many other parts of the world.

[Footnote *: According to Sir H. Davy’s views of the nature of the
muriatic and oxy-muriatic acids, dry muriat of soda is a compound
of sodium and chlorine, for it may be formed by the direct
combination of oxy-muriatic gas and sodium. In his opinion,
therefore, what we commonly call muriat of soda contains neither
soda nor muriatic acid.]

EMILY.

I thought that salts, when solid, were always in the state of crystals; but the common table-salt is in the form of a coarse white powder.

MRS. B.

Crystallisation depends, as you may recollect, on the slow and regular reunion of particles dissolved in a fluid; common sea-salt is only in a state of imperfect crystallisation, because the process by which it is prepared is not favourable to the formation of regular crystals. But if you dissolve it, and afterwards evaporate the water slowly, you will obtain a regular crystallisation.

_Muriat of ammonia_ is another combination of this acid, which we have already mentioned as the principal source from which ammonia is derived.

I can at once show you the formation of this salt by the immediate combination of muriatic acid with ammonia. --These two glass jars contain, the one muriatic acid gas, the other ammoniacal gas, both of which are perfectly invisible--now, if I mix them together, you see they immediately form an opake white cloud, like smoke. --If a thermometer was placed in the jar in which these gases are mixed, you would perceive that some heat is at the same time produced.

EMILY.

The effects of chemical combinations are, indeed, wonderful! --How extraordinary it is that two invisible bodies should become visible by their union!

MRS. B.

This strikes you with astonishment, because it is a phenomenon which nature seldom exhibits to our view; but the most common of her operations are as wonderful, and it is their frequency only that prevents our regarding them with equal admiration. What would be more surprising, for instance, than combustion, were it not rendered so familiar by custom?

EMILY.

That is true. --But pray, Mrs. B., is this white cloud the salt that produces ammonia? How different it is from the solid muriat of ammonia which you once showed us!

MRS. B.

It is the same substance which first appears in the state of vapour, but will soon be condensed by cooling against the sides of the jar, in the form of very minute crystals.

We may now proceed to the _oxy-muriats_. In this class of salts the _oxy-muriat of potash_ is the most worthy of our attention, for its striking properties. The acid, in this state of combination, contains a still greater proportion of oxygen than when alone.

CAROLINE.

But how can the oxy-muriatic acid acquire an increase of oxygen by combining with potash?

MRS. B.

It does not really acquire an additional quantity of oxygen, but it loses some of the muriatic acid, which produces the same effect, as the acid which remains is proportionably super-oxygenated.*

If this salt be mixed, and merely rubbed together with sulphur, phosphorus, charcoal, or indeed any other combustible, it explodes strongly.

[Footnote *: According to Sir H. Davy’s new views, just explained,
oxy-muriat of potash is a compound of chlorine with oxyd of
potassium.]

CAROLINE.

Like gun-powder, I suppose, it is suddenly converted into elastic fluids?

MRS. B.

Yes; but with this remarkable difference, that no increase of temperature, any further than is produced by gentle friction, is required in this instance. Can you tell me what gases are generated by the detonation of this salt with charcoal?

EMILY.

Let me consider . . . . . The oxy-muriatic acid parts with its excess of oxygen to the charcoal, by which means it is converted into muriatic acid gas; whilst the charcoal, being burnt by the oxygen, is changed to carbonic acid gas. --What becomes of the potash I cannot tell.

MRS. B.

That is a fixed product which remains in the vessel.

CAROLINE.

But since the potash does not enter into the new combinations, I do not understand of what use it is in this operation. Would not the oxy-muriatic acid and the charcoal produce the same effect without it?

MRS. B.

No; because there would not be that very great concentration of oxygen which the combination with the potash produces, as I have just explained.

I mean to show you this experiment, but I would advise you not to repeat it alone; for if care be not taken to mix only very small quantities at a time, the detonation will be extremely violent, and may be attended with dangerous effects. You see I mix an exceedingly small quantity of the salt with a little powdered charcoal, in this Wedgwood mortar, and rub them together with the pestle--

CAROLINE.

Heavens! How can such a loud explosion be produced by so small a quantity of matter?

MRS. B.

You must consider that an extremely small quantity of solid substance may produce a very great volume of gases; and it is the sudden evolution of these which occasions the sound.

EMILY.

Would not oxy-muriat of potash make stronger gunpowder than nitrat of potash?

MRS. B.

Yes; but the preparation, as well as the use of this salt, is attended with so much danger, that it is never employed for that purpose.

CAROLINE.

There is no cause to regret it, I think; for the common gunpowder is quite sufficiently destructive.

MRS. B.

I can show you a very curious experiment with this salt; but it must again be on condition that you will never attempt to repeat it by yourselves. I throw a small piece of phosphorus into this glass of water; then a little oxy-muriat of potash; and, lastly, I pour in (by means of this funnel, so as to bring it in contact with the two other ingredients at the bottom of the glass) a small quantity of sulphuric acid--

CAROLINE.

This is, indeed, a beautiful experiment! The phosphorus takes fire and burns from the bottom of the water.

EMILY.

How wonderful it is to see flame bursting out under water, and rising through it! Pray, how is this accounted for?

MRS. B.

Cannot you find it out, Caroline?

EMILY.

Stop--I think I can explain it. Is it not because the sulphuric acid decomposes the salt by combining with the potash, so as to liberate the oxy-muriatic acid gas by which the phosphoric is set on fire?

MRS. B.

Very well, Emily; and with a little more reflection you would have discovered another concurring circumstance, which is, that an increase of temperature is produced by the mixture of the sulphuric acid and water, which assists in promoting the combustion of the phosphorus.

I must, before we part, introduce to your acquaintance the newly-discovered substance IODINE, which you may recollect we placed next to oxygen and chlorine in our table of simple bodies.

CAROLINE.

Is this also a body capable of maintaining combustion like oxygen and chlorine?

MRS. B.

It is; and although it does not so generally disengage light and heat from inflammable bodies, as oxygen and chlorine do, yet it is capable of combining with most of them; and sometimes, as in the instance of potassium and phosphorus, the combination is attended with an actual appearance of light and heat.

CAROLINE.

But what sort of a substance is iodine: what is its form, and colour?

MRS. B.

It is a very singular body, in many respects. At the ordinary temperature of the atmosphere, it commonly appears in the form of blueish black crystalline scales, such as you see in this tube.

CAROLINE.

They shine like black lead, and some of the scales have the shape of lozenges.

MRS. B.

That is actually the form which the crystals of iodine often assume. But if we heat them gently, by holding the tube over the flame of a candle, see what a change takes place in them.

CAROLINE.

How curious! They seem to melt, and the tube immediately fills with a beautiful violet vapour. But look, Mrs. B., the same scales are now appearing at the other end of the tube.

MRS. B.

This is in fact a sublimation of iodine, from one part of the tube to another; but with this remarkable peculiarity, that, while in the gaseous state, iodine assumes that bright violet colour, which, as you may already perceive, it loses as the tube cools, and the substance resumes its usual solid form. --It is from the violet colour of the gas that iodine has obtained its name.

CAROLINE.

But how is this curious substance obtained?

MRS. B.