Chapter X: Sodium Chloride--Berthollet's Laws--Hydrochloric Acid (1)

In the preceding chapters we have become acquainted with the most important properties of the four elements, hydrogen, oxygen, nitrogen, and carbon. They are sometimes termed the _organogens_, because they enter into the composition of organic substances. Their mutual combinations may serve as types for all other chemical compounds--that is, they present the same atomic relations (types, forms, or grades of combinations) as those in which the other elements also combine together.

Hydrogen, HH, or, in general, HR.
Water, H_{2}O, " " H_{2}R.
Ammonia, H_{3}N, " " H_{3}R.
Marsh gas, H_{4}C, " " H_{4}R.

One, two, three, and four atoms of hydrogen enter into these molecules for one atom of another element. No compounds of one atom of oxygen with three or four atoms of hydrogen are known; hence the atom of oxygen does not possess certain properties which are found in the atoms of carbon and nitrogen.

The faculty of an element to form a compound of definite composition with hydrogen (or an element analogous to it) gives the possibility of foretelling the composition of many other of its compounds. Thus, if we know that an element, M, combines with hydrogen, forming, by preference, a gaseous substance such as HM, but not forming H_{2}M, H_{3}M, H_{n}M_{m}, then we must conclude, on the basis of the law of substitution, that this element will give compounds M_{2}O, M_{3}N, MHO, MH_{3}C, &c. Chlorine is an example of this kind. If we know that another element, R, like oxygen, gives with hydrogen a molecule H_{2}R, then we may expect that it will form compounds similar to hydrogen peroxide, the metallic oxides, carbonic anhydride, or carbonic oxide, and others. Sulphur is an instance of this kind. Hence the elements may be classified according to their resemblance to hydrogen, oxygen, nitrogen, and carbon, and in conformity with this analogy it is possible to foretell, if not the properties (for example, the acidity or basicity), at any rate the composition,[1] of some of their compounds. This forms the substance of _the conception of the valency or atomicity of the elements_. Hydrogen is taken as the representative of the univalent elements, giving compounds, RH, R(OH), R_{2}O, RCl, R_{3}N, R_{4}C, &c. Oxygen, in that form in which it gives water, is the representative of the bivalent elements, forming RH_{2}, RO, RCl_{2}, RHCl, R(OH)Cl, R(OH)_{2}, R_{2}C, RCN, &c. Nitrogen in ammonia is the representative of the trivalent elements, giving compounds RH_{3}, R_{2}O_{3}, R(OH)_{3}, RCl_{3}, RN, RHC, &c. In carbon are exemplified the properties of the quadrivalent elements, forming RH_{4}, RO_{2}, RO(OH)_{2}, R(OH)_{4}, RHN, RCl_{4}, RHCl_{3}, &c. We meet with these _forms of combination_, or degrees of union of atoms, in all other elements, some being analogous to hydrogen, others to oxygen, and others to nitrogen or to carbon. But besides these quantitative analogies or resemblances, which are foretold by the law of substitution (Chapter VI.), there exist among the elements qualitative analogies and relations which are not fully seen in the compounds of the elements which have been considered, but are most distinctly exhibited in the formation of bases, acids, and salts of different types and properties. Therefore, for a complete study of the nature of the elements and their compounds it is especially important to become acquainted with the salts, as substances of a peculiar character, and with the corresponding acids and bases. Common table salt, or sodium chloride, NaCl, may in every respect be taken as a type of salts in general, and we will therefore pass to the consideration of this substance, and of hydrochloric acid, and of the base sodium hydroxide, formed by the non-metal chlorine and the metal sodium, which correspond with it.

[1] But it is impossible to foretell all the compounds formed by an
element from its atomicity or valency, because the atomicity of the
elements is variable, and furthermore this variability is not
identical for different elements. In CO_{2}, COX_{2}, CH_{4}, and
the multitude of carbon compounds corresponding with them, the C is
quadrivalent, but in CO either the carbon must be taken as bivalent
or the atomicity of oxygen be accounted as variable. Moreover,
carbon is an example of an element which preserves its atomicity to
a greater degree than most of the other elements. Nitrogen in
NH_{3}, NH_{2}(OH), N_{2}O_{3}, and even in CNH, must be considered
as trivalent, but in NH_{4}Cl, NO_{2}(OH), and in all their
corresponding compounds it is necessarily pentavalent. In N_{2}O,
if the atomicity of oxygen = 2, nitrogen has an uneven atomicity
(1, 3, 5), whilst in NO it is bivalent. If sulphur be bivalent,
like oxygen, in many of its compounds (for example, H_{2}S,
SCl_{2}, KHS, &c.), then it could not be foreseen from this that it
would form SO_{2}, SO_{3}, SCl_{4}, SOCl_{2}, and a series of
similar compounds in which its atomicity must be acknowledged as
greater than 2. Thus SO_{2}, sulphurous anhydride, has many points
in common with CO_{2}, and if carbon be quadrivalent then the S in
SO_{2} is quadrivalent. Therefore the principle of atomicity
(valency) of the elements cannot be considered established as the
basis for the study of the elements, although it gives an easy
method of grasping many analogies. I consider the four following as
the chief obstacles to acknowledging the atomicity of the elements
as a primary conception for the consideration of the properties of
the elements: 1. Such univalent elements as H, Cl, &c., appear in a
free state as molecules H_{2}, Cl_{2}, &c., and are consequently
like the univalent radicles CH_{3}, OH, CO_{2}H, &c., which, as
might be expected, appear as C_{2}H_{6}, O_{2}H_{2},
C_{2}O_{4}H_{2} (ethane, hydrogen peroxide, oxalic acid), whilst on
the other hand, potassium and sodium (perhaps also iodine at a high
temperature) contain only one atom, K, Na, in the molecule in a
free state. Hence it follows that _free affinities_ may exist.
Granting this, nothing prevents the assumption that free affinities
exist in all unsaturated compounds; for example, two free
affinities in NH_{3}. If such instances of free affinities be
admitted, then all the possible advantages to be gained by the
application of the doctrine of atomicity (valency) are lost. 2.
There are instances--for example, Na_{2}H--where univalent elements
are combined in molecules which are more complex than R_{2}, and
form molecules, R_{3}, R_{4}, &c.; this may again be either taken
as evidence of the existence of free affinities, or else
necessitates such primary univalent elements as sodium and hydrogen
being considered as variable in their atomicity. 3. The periodic
system of the elements, with which we shall afterwards become
acquainted, shows that there is a law or rule for the variation of
the forms of oxygen and hydrogen compounds; chlorine is univalent
with respect to hydrogen, and septavalent with respect to oxygen;
sulphur is bivalent to hydrogen, and sexavalent to oxygen;
phosphorus is trivalent to hydrogen and pentavalent in respect to
oxygen--the sum is in every case equal to 8. Only carbon and its
analogues (for example, silicon) are quadrivalent to both hydrogen
and oxygen. Hence the power of the elements to change their
atomicity is an essential part of their nature, and therefore
constant valency cannot he considered as a fundamental property. 4.
Crystallo-hydrates (for instance, NaCl,2H_{2}O, or NaBr,2H_{2}O),
double salts (such as PtCl_{4},2KCl,H_{2}SiF_{6}, &c.), and similar
complex compounds (and, according to Chap. I., solutions also)
demonstrate the capacity not only of the elements themselves, but
also of their saturated and limiting compounds, of entering into
further combination. Therefore the admission of a definite limited
atomicity of the elements includes in itself an admission of
limitation which is not in accordance with the nature of chemical
reactions.

_Sodium chloride_, NaCl, the familiar table salt, occurs, although in very small quantities, in all the primary formations of the earth's crust,[2] from which it is washed away by the atmospheric waters; it is contained in small quantities in all waters flowing through these formations, and is in this manner conveyed to the oceans and seas. The immense mass of salt in the oceans has been accumulated by this process from the remote ages of the earth's creation, because the water has evaporated from them while the salt has remained in solution. The salt of sea water serves as the source not only for its direct extraction, but also for the formation of other masses of workable salt, such as rock salt, and of saline springs and lakes.[2 bis]

[2] The primary formations are those which do not bear any distinct
traces of having been deposited from water (have not a stratified
formation and contain no remains of animal or vegetable life),
occur under the sedimentary formations of the earth, and are
everywhere uniform in composition and structure, the latter being
generally distinctly crystalline. If it be assumed that the earth
was originally in a molten condition, the first primary formations
are those which formed the first solid crust of the earth. But even
with this hypothesis of the earth's origin, it is necessary to
admit that the first aqueous deposits must have caused a change in
the original crust of the earth, and therefore under the head of
primary formations must be understood the most ancient of the
products of decomposition (mostly by atmospheric, aqueous, and
organic agency, &c.), from which all the rocks and substances of
the earth's surface have arisen. In speaking of the origin of one
or another substance, we can only, on the basis of facts, descend
to the primary formations, of which granite, gneiss, and trachyte
may be taken as examples.

[2 bis] Chloride of sodium has been found to occur in the atmosphere in
the form of a fine dust; in the lower strata it is present in
larger quantities than in the upper, so that the rain water falling
on mountains contains less NaCl than that falling in valleys. Müntz
(1891) found that a litre of rain water collected on the summit of
the Pic du Midi (2,877 metres above the sea level) contained 0·34
milligram of chloride of sodium, while a litre of rain collected
from the valley contained 2·5-7·6 milligrams.

The extraction of salt _from sea water_ is carried on in several ways. In southern climes, especially on the shores of the Atlantic Ocean and the Mediterranean and Black Seas, the summer heats are taken advantage of. A convenient low-lying sea shore is chosen, and a whole series of basins, communicating with each other, are constructed along it. The upper of these basins are filled with sea water by pumping, or else advantage is taken of high tides. These basins are sometimes separated from the sea by natural sand-banks (limans) or by artificial means, and in spring the water already begins to evaporate considerably. As the solution becomes more concentrated, it is run into the succeeding basins, and the upper ones are supplied with a fresh quantity of sea water, or else an arrangement is made enabling the salt water to flow by degrees through the series of basins. It is evident that the beds of the basins should be as far as possible impervious to water, and for this purpose they are made of beaten clay. The crystals of salt begin to separate out when the concentration attains 28 p.c. of salt (which corresponds to 28° of Baumé's hydrometer). They are raked off, and employed for all those purposes to which table salt is applicable. In the majority of cases only the first half of the sodium chloride which can be separated from the sea water is extracted, because the second half has a bitter taste from the presence of magnesium salts which separate out together with the sodium salt. But in certain localities--as, for instance, in the estuary of the Rhone, on the island of Camarga[3]--the evaporation is carried on to the very end, in order to obtain those magnesium and potassium salts which separate out at the end of the evaporation of sea water. Various salts are separated from sea water in its evaporation. From 100 parts of sea water there separates out, by natural and artificial evaporation, about one part of tolerably pure table salt at the very commencement of the operation; the total amount held in solution being about 2-1/2 p.c. The remaining portion separates out intermixed with the bitter salts of magnesium which, owing to their solubility and the small amount in which they are present (less than 1 p.c.), only separate out, in the first crystallisations, in traces. Gypsum, or calcium sulphate, CaSO_{4},2H_{2}O, because of its sparing solubility, separates together with or even before the table salt. When about half of the latter has separated, then a mixture of table salt and magnesium sulphate separates out, and on still further evaporation the chlorides of potassium and magnesium begin to separate in a state of combination, forming the double salt KMgCl_{3},6H_{2}O, which occurs in nature as _carnallite_.[4] After the separation of this salt from sea water, there remains a mother liquor containing a large amount of magnesium chloride in admixture with various other salts.[5] The extraction of sea salt is usually carried on for the purpose of procuring table salt, and therefore directly it begins to separate mixed with a considerable proportion[6] of magnesium salts (when it acquires a bitter taste) the remaining liquor is run back into the sea.

[3] The extraction of the potassium salts (or so-called summer salts)
was carried on at the Isle of Camarga about 1870, when I had
occasion to visit that spot. At the present time the deposits of
Stassfurt provide a much cheaper salt, owing to the evaporation and
separation of the salt being carried on there by natural means and
only requiring a treatment and refining, which is also necessary in
addition for the 'summer salt' obtained from sea-water.

[4] The double salt KCl,MgCl_{2} is a crystallohydrate of KCl and
MgCl_{2}, and is only formed from solutions containing an excess of
magnesium chloride, because water decomposes this double salt,
extracting the more soluble magnesium chloride from it.

[5] Owing to the fundamental property of salts of interchanging their
metals, it cannot be said that sea water contains this or that
salt, but only that it contains certain amounts of certain metals M
(univalent like Na and K, and bivalent like Mg and Ca), and haloids
X (univalent like Cl, Br, and bivalent like SO_{4}, CO_{3}), which
are disposed in every possible kind of grouping; for instance, K as
KCl, KBr, K_{2}SO_{4}, Mg as MgCl_{2}, MgBr_{2}, MgSO_{4}, and so
on for all the other metals. In evaporation different salts
separate out consecutively only because they reach saturation. A
proof of this may be seen in the fact that a solution of a mixture
of sodium chloride and magnesium sulphate (both of which salts are
obtained from sea water, as was mentioned above), when evaporated,
deposits crystals of these salts, but when refrigerated (if the
solution be sufficiently saturated) the salt Na_{2}SO_{4},10H_{2}O
is first deposited because it is the first to arrive at saturation
at low temperatures. Consequently this solution contains MgCl_{2}
and Na_{2}SO_{4}, besides MgSO_{4} and NaCl. So it is with sea
water.

[6] The salt extracted from water is piled up in heaps and left exposed
to the action of rain water, which purifies it, owing to the water
becoming saturated with sodium chloride and then no longer
dissolving it, but washing out the impurities.

The same process which is employed for artificially obtaining salt in a crystalline form from sea water has been repeatedly accomplished during the geological evolution of the earth on a gigantic scale; upheavals of the earth have cut off portions of the sea from the remainder (as the Dead Sea was formerly a part of the Mediterranean, and the Sea of Aral of the Caspian), and their water has evaporated and formed (if the mass of the inflowing fresh water were less than that of the mass evaporated) deposits of _rock salt_. It is always accompanied by gypsum, because the latter is separated from sea water with or before the sodium chloride. For this reason rock salt may always be looked for in those localities where there are deposits of gypsum. But inasmuch as the gypsum remains on the spot where it has been deposited (as it is a sparingly soluble salt), whilst the rock salt (as one which is very soluble) may be washed away by rain or fresh running water, it may sometimes happen that although gypsum is still found there may be no salt; but, on the other hand, where there is rock salt there will always be gypsum. As the geological changes of the earth's surface are still proceeding at the present day, so in the midst of the dry land salt lakes are met with, which are sometimes scattered over vast districts formerly covered by seas now dried up. Such is the origin of many of the salt lakes about the lower portions of the Volga and in the Kirghiz steppes, where at a geological epoch preceding the present the Aralo-Caspian Sea extended. Such are the Baskunchaksky (in the Government of Astrakhan, 112 square kilometres superficial area), the Eltonsky (140 versts from the left bank of the Volga, and 200 square kilometres in superficial area), and upward of 700 other salt lakes lying about the lower portions of the Volga. In those in which the inflow of fresh water is less than that yearly evaporated, and in which the concentration of the solution has reached saturation, the _self-deposited_ salt is found already deposited on their beds, or is being yearly deposited during the summer months. Certain limans, or sea-side lakes, of the Azoff Sea are essentially of the same character--as, for instance, those in the neighbourhood of Henichesk and Berdiansk. The saline soils of certain Central Asian steppes, which suffer from a want of atmospheric fresh water, are of the same origin. Their salt originally proceeded from the salt of seas which previously covered these localities, and has not yet been washed away by fresh water. The main result of the above-described process of nature is the formation of masses of rock salt, which are, however, being gradually washed away by the subsoil waters flowing in their neighbourhood, and afterwards rising to the surface in certain places as _saline springs_, which indicate the presence of masses of deposited rock salt in the depths of the earth. If the subsoil water flows along a stratum of salt for a sufficient length of time it becomes saturated; but in flowing in its further course along an impervious stratum (clay) it becomes diluted by the fresh water leaking through the upper soil, and therefore the greater the distance of a saline spring from the deposit of rock salt, the poorer will it be in salt. A perfectly saturated brine, however, may be procured from the depths of the earth by means of bore-holes. The deposits of rock salt themselves, which are sometimes hidden at great depths below the earth's strata, may be discovered by the guidance of bore-holes and the direction of the strata of the district. Deposits of rock salt, about 35 metres thick and 20 metres below the surface, were discovered in this manner in the neighbourhood of Brianstcheffky and Dekonoffky, in the Bakhmut district of the Government of Ekaterinoslav. Large quantities of most excellent rock salt are now (since 1880) obtained from these deposits, whose presence was indicated by the neighbouring salt springs (near Slaviansk and Bakhmut) and by bore-holes which had been sunk in these localities for procuring strong (saturated) brines. But the Stassfurt deposits of rock salt near Magdeburg in Germany are celebrated as being the first discovered in this manner, and for their many remarkable peculiarities.[7] The plentiful distribution of saline springs in this and the neighbouring districts suggested the presence of deposits of rock salt in the vicinity. Deep bore-holes sunk in this locality did in fact give a richer brine--even quite saturated with salt. On sinking to a still greater depth, the deposits of salt themselves were at last arrived at. But the first deposit which was met with consisted of a bitter salt unfit for consumption, and was therefore called refuse salt (_Abraumsalz_). On sinking still deeper vast beds of real rock salt were struck. In this instance the presence of these upper strata containing salts of potassium, magnesium, and sodium is an excellent proof of the formation of rock salt from sea water. It is very evident that not only a case of evaporation to the end--as far, for instance, as the separation of carnallite--but also the preservation of such soluble salts as separate out from sea water after the sodium chloride, must be a very exceptional phenomenon, which is not repeated in all deposits of rock salt. The Stassfurt deposits therefore are of particular interest, not only from a scientific point of view, but also because they form a rich source of potassium salts which have many practical uses.[7 bis]

[7] When the German savants pointed out the exact locality of the
Stassfurt salt-beds and their depth below the surface, on the basis
of information collected from various quarters respecting
bore-holes and the direction of the strata, and when the borings,
conducted by the Government, struck a salt-bed which was bitter and
unfit for use, there was a great outcry against science, and the
doubtful result even caused the cessation of the further work of
deepening the shafts. It required a great effort to persuade the
Government to continue the work. Now, when the pure salt
encountered below forms one of the important riches of Germany, and
when those 'refuse salts' have proved to be most valuable (as a
source of potassium and magnesium), we should see in the
utilisation of the Stassfurt deposits one of the conquests of
science for the common welfare.

[7 bis] In Western Europe, deposits of rock salt have long been known
at Wieliczka, near Cracow, and at Cardona in Spain. In Russia the
following deposits are known: (_a_) the vast masses of rock salt (3
square kilometres area and up to 140 metres thick) lying directly
on the surface of the earth at Iletzky Zastchit, on the left bank
of the river Ural, in the Government of Orenburg; (_b_) the
Chingaksky deposit, 90 versts from the river Volga, in the
Enotaeffsky district of the Government of Astrakhan; (_c_) the
Kulepinsky (and other) deposits (whose thickness attains 150
metres), on the Araks, in the Government of Erivan in the Caucasus;
(_d_) the Katchiezmansky deposit in the province of Kars; (_e_) the
Krasnovodsky deposit in the Trans-Caspian province; and (_f_) the
Bardymkulsky salt mines in Kokhand.

A saturated brine, formed by the continued contact of subsoil water with rock salt, is extracted by means of bore-holes, as, for instance, in the Governments of Perm, Kharkoff, and Ekaterinoslav. Sometimes, as at Berchtesgaden (and at Hallein) in Austria, spring water is run on to underground beds of rock salt containing much clay.

If a saline spring, or the salt water pumped from bore-holes, contains but little salt, then the first concentration of the natural solution is not carried on by the costly consumption of fuel, but by the cheaper method of evaporation by means of the wind. For this purpose so-called graduators are constructed: they consist of long and lofty sheds, which are sometimes several versts long, and generally extend in a direction at right angles to that of the usual course of the wind in the district. These sheds are open at the sides, and are filled with brushwood as shown in fig. 64. Troughs, A B, C D, into which the salt water is pumped, run along the top. On flowing from these troughs, through the openings, _a_, the water spreads over the brushwood and distributes itself in a thin layer over it, so that it presents a very large surface for evaporation, in consequence of which it rapidly becomes concentrated in warm or windy weather. After trickling over the brushwood, the solution collects in a reservoir under the graduator, whence it is usually pumped up by the pumps P P´, and again run a second and third time through the graduator, until the solution reaches a degree of concentration at which it becomes profitable to extract the salt by direct heating. Generally the evaporation in the graduator is not carried beyond a concentration of 12 to 15 parts of salt in 100 parts of solution. Strong natural solutions of salt, and also the graduated solutions, are evaporated in large shallow metallic vessels, which are either heated by the direct action of the flame from below or from above. These vessels are made of boiler plate, and are called salt-pans. Various means are employed for accelerating the evaporation and for economising fuel, which are mainly based on an artificial draught to carry off the steam as it is formed, and on subjecting the saline solution to a preliminary heating by the waste heat of the steam and furnace gases. Furthermore, the first portions of the salt which crystallise out in the salt-pans are invariably contaminated with gypsum, since the waters of saline springs always contain this substance. It is only the portions of the salt which separate later that are distinguished by their great purity. The salt is ladled out as it is deposited, left to drain on inclined tables and then dried, and in this manner the so-called bay salt is obtained. Since it has become possible to discover the saline deposits themselves, the extraction of table salt from the water of saline springs by evaporation, which previously was in general use, has begun to be disused, and is only able to hold its ground in places where fuel is cheap.

In order to understand the full importance of the extraction of salt, it need only be mentioned that on the average 20 lbs. of table salt are consumed yearly per head of population, directly in food or for cattle. In those countries where common salt is employed in technical processes, and especially in England, almost an equal quantity is consumed in the production of substances containing chlorine and sodium, and especially in the manufacture of washing soda, &c., and of chlorine compounds (bleaching powder and hydrochloric acid). The yearly production of salt in Europe amounts to as much as 7-1/2 million tons.

Although certain lumps of rock salt and crystals of bay salt sometimes consist of almost pure sodium chloride, still the ordinary commercial salt contains various impurities, the most common of which are magnesium salts. If the salt be pure, its solution gives no precipitate with sodium carbonate, Na_{2}CO_{3}, showing the absence of magnesium salts, because magnesium carbonate, MgCO_{3}, is insoluble in water. Rock salt, which is ground for use, generally contains also a considerable admixture of clay and other insoluble impurities.[8] For ordinary use the bulk of the salt obtained can be employed directly without further purification; but some salts are purified by solution and crystallisation of the solution after standing, in which case the evaporation is not carried on to dryness, and the impurities remain in the _mother liquor_ or in the sediment. When perfectly pure salt is required for chemical purposes it is best obtained as follows: a saturated solution of table salt is prepared, and hydrochloric acid gas is passed through it; this precipitates the sodium chloride (which is not soluble in a strong solution of hydrochloric acid), while the impurities remain in solution. By repeating the operation and fusing the salt (when adhering hydrochloric acid is volatilised) a pure salt is obtained, which is again crystallised from its solution by evaporation.[9]

[8] The fracture of rock salt generally shows the presence of
interlayers of impurities which are sometimes very small in weight,
but visible owing to their refraction. In the excellently laid out
salt mines of Briansk I counted (1888), if my memory does not
deceive me, on an average ten interlayers per metre of thickness,
between which the salt was in general very pure, and in places
quite transparent. If this be the case, then there would be 350
interlayers for the whole thickness (about 35 metres) of the bed.
They probably correspond with the yearly deposition of the salt. In
this case the deposition would have extended over more than 300
years. This should be observable at the present day in lakes where
the salt is saturated and in course of deposition.

[9] My own investigations have shown that not only the sulphates, but
also the potassium salts, are entirely removed by this method.

Pure sodium chloride, in well-defined crystals (slowly deposited at the bottom of the liquid) or in compact masses (in which form rock salt is sometimes met with), is a colourless and transparent substance resembling, but more brittle and less hard than, glass.[10] Common salt always crystallises in the cubic system, most frequently in _cubes_, and more rarely in octahedra. Large transparent cubes of common salt, having edges up to 10 centimetres long, are sometimes found in masses of rock salt.[11] When evaporated in the open the salt often separates out on the surface[12] as cubes, which grow on to each other in the form of pyramidal square funnels. In still weather, these clusters are able to support themselves on the surface of the water for a long time, and sometimes go on increasing to a considerable extent, but they sink directly the water penetrates inside them. Salt fuses to a colourless liquid (sp. gr. 1·602, according to Quincke) at 851° (V. Meyer); if pure it solidifies to a non-crystalline mass, and if impure to an opaque mass whose surface is not smooth. In fusing, sodium chloride commences to volatilise (its weight decreases) and at a white heat it volatilises with great ease and completely; but at the ordinary temperature it may, like all ordinary salts, be considered as non-volatile, although as yet no exact experiments have been made in this direction.

[10] According to the determinations of Klodt, the Briansk rock salt
withstands a pressure of 340 kilograms per square centimetre,
whilst glass withstands 1,700 kilos. In this respect salt is twice
as secure as bricks, and therefore immense masses may be extracted
from underground workings with perfect safety, without having
recourse to brickwork supports, merely taking advantage of the
properties of the salt itself.

[11] To obtain well-formed crystals, a saturated solution is mixed with
ferric chloride, several small crystals of sodium chloride are
placed at the bottom, and the solution is allowed to evaporate
slowly in a vessel with a loose-fitting cover. Octahedral crystals
are obtained by the addition of borax, urea, &c., to the solution.
Very fine crystals are formed in a mass of gelatinous silica.

[12] If a solution of sodium chloride be slowly heated from above,
where the evaporation takes place, then the upper layer will
become saturated before the lower and cooler layers, and therefore
crystallisation will begin on the surface, and the crystals first
formed will float, having also dried from above, on the surface
until they become quite soaked. Being heavier than the solution
the crystals are partially immersed under it, and the following
crystallisation, also proceeding on the surface, will only form
crystals along the side of the original crystals. A funnel is
formed in this manner. It will be borne on the surface like a boat
(if the liquid be quiescent), because it will grow more from the
upper edges. We can thus understand this at first sight strange
funnel form of crystallisation of salt. In explanation why the
crystallisation under the above conditions begins at the surface
and not at the lower layers, it must be mentioned that the
specific gravity of a crystal of sodium chloride = 2·16, and that
of a solution saturated at 25° contains 26·7 p.c. of salt and has
a specific gravity at 25°/4° of 1·2004; at 15° a saturated
solution contains 26·5 p.c. of salt and has a sp. gr. 1·203 at
15°/4°. Hence a solution saturated at a higher temperature is
specifically lighter, notwithstanding the greater amount of salt
it contains. With many substances _surface crystallisation_ cannot
take place because their solubility increases more rapidly with
the temperature than their specific gravity decreases. In this
case the saturated solution will always be in the lower layers,
where also the crystallisation will take place. Besides which it
may be added that as a consequence of the properties of water and
solutions, when they are heated from above (for instance, by the
sun's rays), the warmer layers being the lightest remain above,
whilst when heated from below they rise to the top. For this
reason the water at great depths below the surface is always cold,
which has long been known. These circumstances, as well as those
observed by Soret (Chapter I., Note 19), explain the great
differences of density and temperature, and in the amount of salts
held in the oceans at different latitudes (in polar and tropical
climes) and at various depths.

A saturated[13] solution of table salt (containing 26·4 p.c.) has at the ordinary temperature a specific gravity of about 1·2. The specific gravity of the crystals is 2·167 (17°). The salt which separates out at the ordinary and higher temperatures contains no water of crystallisation;[14] but if the crystals are formed at a low temperature, especially from a saturated solution cooled to -12°, then they present a prismatic form, and contain two equivalents of water, NaCl,2H_{2}O. At the ordinary temperature these crystals split up into sodium chloride and its solution.[15] Unsaturated solutions of table salt when cooled below 0° give[16] crystals of ice, but when the solution has a composition NaCl,10H_{2}O it solidifies completely at a temperature of -23°. A solution of table salt saturated at its boiling point boils at about 109°, and contains about 42 parts of salt per 100 parts of water.

[13] By combining the results of Poggiale, Müller, and Karsten (they
are evidently more accurate than those of Gay-Lussac and others) I
found that a saturated solution at _t_°, from 0° to 108°, contains
35·7 + 0·024_t_ + 0·0002_t_^2 grams of salt per 100 grams of
water. This formula gives a solubility at 0° = 35·7 grams (= 26·3
p.c.), whilst according to Karsten it is 36·09, Poggiale 35·5, and
Müller 35·6 grams.

[14] Perfectly pure _fused_ salt is not hygroscopic, according to
Karsten, whilst the crystallised salt, even when quite pure,
attracts as much as 0·6 p.c. of water from moist air, according to
Stas. (In the Briansk mines, where the temperature throughout the
whole year is about +10°, it may be observed, as Baron Klodt
informed me, that in the summer during damp weather the walls
become moist, while in winter they are dry).

If the salt contain impurities--such as magnesium sulphate, &c.--it
is more hygroscopic. If it contain any magnesium chloride, it
partially deliquesces in a damp atmosphere. The crystallised and
not perfectly pure salt decrepitates when heated, owing to its
containing water. The pure salt, and also the transparent rock
salt, or that which has been once fused, does not decrepitate.
Fused sodium chloride shows a faint alkaline reaction to litmus,
which has been noticed by many observers, and is due to the
presence of sodium oxide (probably by the action of the oxygen of
the atmosphere). According to A. Stcherbakoff very sensitive
litmus (washed in alcohol and neutralised with oxalic acid) shows
an alkaline reaction even with the crystallised salt.

It may be observed that rock salt sometimes contains cavities
filled with a colourless liquid. Certain kinds of rock salt emit
an odour like that of hydrocarbons. These phenomena have as yet
received very little attention.

[15] By cooling a solution of table salt saturated at the ordinary
temperature to -15°, I obtained first of all well-formed tabular
(six-sided) crystals, which when warmed to the ordinary
temperature disintegrated (with the separation of anhydrous sodium
chloride), and then prismatic needles up to 20 mm. long were
formed from the same solution. I have not yet investigated the
reason of the difference in crystalline form. It is known
(Mitscherlich) that NaI,2H_{2}O also crystallises either in plates
or prisms. Sodium bromide also crystallises with 2H_{2}O at the
ordinary temperature.

[16] Notwithstanding the great simplicity (Chapter I., Note 49) of the
observations on the formation of ice from solution, still even for
sodium chloride they cannot yet be considered as sufficiently
harmonious. According to Blagden and Raoult, the temperature of
the formation of ice from a solution containing _c_ grams of salt
per 100 grams of water =-0·6_c_ to _c_ = 10, according to Rosetti
=-0·649_c_ to _c_ = 8·7, according to De Coppet (to _c_ = 10)
=-0·55_c_-0·006_c_^2, according to Karsten (to _c_ = 10)-0·762_c_
+ 0·0084_c_^2, and according to Guthrie a much lower figure. By
taking Rosetti's figure and applying the rule given in Chapter I.,
Note 49 we obtain--

_i_ - 0·649 × 58·5/18·5 = 2·05.

Pickering (1893) gives for _c_ = 1-0·603, for _c_ = 2-1·220; that
is (_c_ up to 2·7) about -(0·600 + 0·005_c_)_c_.

The data for strong solutions are not less contradictory. Thus
with 20 p.c. of salt, ice is formed at -14·4° according to
Karsten,-17° according to Guthrie,-17·6° according to De Coppet.
Rüdorff states that for strong solutions the temperature of the
formation of ice descends in proportion to the contents of the
compound, NaCl,2H_{2}O (per 100 grams of water) by 0°·342 per 1
gram of salt, and De Coppet shows that there is no
proportionality, in a strict sense, for either a percentage of
NaCl or of NaCl,2H_{2}O.

Of all its physical properties the specific gravity of solutions of sodium chloride is the one which has been the most fully investigated. A comparison of all the existing determinations of the specific gravity of solutions of NaCl[17] at 15° (in vacuo, taking water at 4° as 10,000), with regard to _p_ (the percentage amount of the salt in solution), show that it is expressed by the equation S_{15} = 9991·6 + 71·17_p_ + 0·2140_p_^2. For instance, for a solution 200H_{2}O + NaCl, in which case _p_ = 1·6, S_{15} = 1·0106. It is seen from the formula that the addition of water produces a contraction.[18] The specific gravity[19] at certain temperatures and concentrations in vacuo referred to water at 4° = 10,000[20] is here given for

0° 15° 30° 100°
_p_ = 5 10372 10353 10307 9922
10 10768 10728 10669 10278
15 11164 11107 11043 10652
20 11568 11501 11429 11043

It should be remarked that Baumé's hydrometer is graduated by taking a 10 p.c. solution of sodium chloride as 10° on the scale, and therefore it gives approximately the percentage amount of the salt in a solution. Common salt is somewhat soluble in alcohol,[21] but it is insoluble in ether and in oils.

[17] A collection of observations on the specific gravity of solutions
of sodium chloride is given in my work cited in Chapter I.,
Note 50.

Solutions of common salt have also been frequently investigated as
regards rate of _diffusion_ (Chapter I.), but as yet there are no
complete data in this respect. It may be mentioned that Graham and
De Vries demonstrated that diffusion in gelatinous masses (for
instance, gelatin jelly, or gelatinous silica) proceeds in the
same manner as in water, which may probably lead to a convenient
and accurate method for the investigation of the phenomena of
diffusion. N. Umoff (Odessa, 1888) investigated the diffusion of
common salt by means of glass globules of definite density. Having
poured water into a cylinder over a layer of a solution of sodium
chloride, he observed during a period of several months the
position (height) of the globules, which floated up higher and
higher as the salt permeated upwards. Umoff found that at a
constant temperature the distances of the globules (that is, the
length of a column limited by layers of definite concentration)
remain constant; that at a given moment of time the concentration,
_q_, of different layers situated at a depth _z_ is expressed by
the equation B-K_z_ = log.(A-_q_), where A, B, and K are
constants; that at a given moment the rate of diffusion of the
different layers is proportional to their depth, &_c._

[18] If _S__{0} be the specific gravity of water, and _S_ the specific
gravity of a solution containing _p_ p.c. of salt, then by mixing
equal weights of water and the solution, we shall obtain a
solution containing 1/2_p_ of the salt, and if it be formed
without contraction, then its specific gravity _x_ will be
determined by the equation 2/_x_ = 1/S_{0} + 1/S, because the
volume is equal to the weight divided by the density. In reality,
the specific gravity is always found to be greater than that
calculated on the supposition of an absence of contraction.

[19] Generally the specific gravity is observed by weighing in air and
dividing the weight in grams by the volume in cubic centimetres,
the latter being found from the weight of water displaced, divided
by its density at the temperature at which the experiment is
carried out. If we call this specific gravity S_{1}, then as a
cubic centimetre of air under the usual conditions weighs about
0·0012 gram, the sp. gr. in a vacuum S = S_{1} + 0·0012 (S_{1}
- 1), if the density of water = 1.

[20] If the sp. gr. S_{2} be found directly by dividing the weight of a
solution by the weight of water at the same temperature and in the
same volume, then the true sp. gr. _S_ referred to water at 4° is
found by multiplying S_{2} by the sp. gr. of water at the
temperature of observation.

[21] According to Schiff 100 grams of alcohol, containing _p_ p.c. by
weight of C_{2}H_{6}O, dissolves at 15°--

_p_ = 10 20 40 60 80
28·5 22·6 13·2 5·9 1·2 grams NaCl.

Common salt gives very few compounds[22] (double salts) and these are very readily decomposed: it is also decomposed with great difficulty and its dissociation is unknown.[23] But it is easily decomposed, both when fused and in solution, by the action of a galvanic current. If the dry salt be fused in a crucible and an electric current be passed through it by immersing carbon or platinum electrodes in it (the positive electrode is made of carbon and the negative of platinum or mercury), it is _decomposed_: the suffocating gas, chlorine, is liberated at the positive pole and metallic sodium at the negative pole. Both of them act on the excess of water at the moment of their evolution; the sodium evolves hydrogen and forms caustic soda, and the chlorine evolves oxygen and forms hydrochloric acid, and therefore on passing a current through a solution of common salt metallic sodium will not be obtained--but oxygen, chlorine, and hydrochloric acid will appear at the positive pole, and hydrogen and caustic soda at the negative pole.[23 bis] Thus salt, like other salts, is decomposed by the action of an electric current into a metal and a haloid (Chapter III.) Naturally, like all other salts, it may be formed from the corresponding base and acid with the separation of water. In fact if we mix caustic soda (base) with hydrochloric acid (acid), table salt is formed, NaHO + HCl = NaCl + H_{2}O.

[22] Amongst the double salts formed by sodium chloride that obtained
by Ditte (1870) by the evaporation of the solution remaining after
heating sodium iodate with hydrochloric acid until chlorine ceases
to be liberated, is a remarkable one. Its composition is
NaIO_{3},NaCl,14H_{2}O. Rammelsberg obtained a similar (perhaps
the same) salt in well-formed crystals by the direct reaction of
both salts.

[23] But it gives sodium in the flame of a Bunsen's burner (see
Spectrum Analysis), doubtless under the reducing action of the
elements carbon and hydrogen. In the presence of an excess of
hydrochloric acid in the flame (when the sodium would form sodium
chloride), no sodium is formed in the flame and the salt does not
communicate its usual coloration.

[23 bis] There is no doubt, however, but that chloride of sodium is
also decomposed in its aqueous solutions with the separation of
sodium, and that it does not simply enter into double
decomposition with the water (NaCl + H_{2}O = NaHO + HCl). This is
seen from the fact that when a saturated solution of NaCl is
rapidly decomposed by an electric current, a large amount of
chlorine appears at the anode and a sodium amalgam forms at the
mercury cathode, which acts but slowly upon the strong solution of
salt. Castner's process for the electrolysis of brine into
chlorine and caustic soda is an application of this method which
has been already worked in England on an industrial scale.

With resspect to the double decompositions of sodium chloride it should be observed that they are most varied, and serve as means of obtaining nearly all the other compounds of sodium and chlorine.

_The double decompositions of sodium chloride_ are almost exclusively based on the possibility of the metal sodium being exchanged for hydrogen and other metals. But neither hydrogen nor any other metal can directly displace the sodium from sodium chloride. This would result in the separation of metallic sodium, which itself displaces hydrogen and the majority of other metals from their compounds, and is not, so far as is known, ever separated by them. The replacement of the sodium in sodium chloride by hydrogen and various metals can only take place by the transference of the sodium into some other combination. If hydrogen or a metal, M, be combined with an element X, then the double decomposition NaCl + MX = NaX + MCl takes place. Such double decompositions take place under special conditions, sometimes completely and sometimes only partially, as we shall endeavour to explain. In order to acquaint ourselves with the double decompositions of sodium chloride, we will follow the methods actually employed in practice to procure compounds of sodium and of chlorine from common salt. For this purpose we will first describe the treatment of sodium chloride with sulphuric acid for the preparation of hydrochloric acid and sodium sulphate. We will then describe the substances obtained from hydrochloric acid and sodium sulphate. Chlorine itself, and nearly all the compounds of this element, may be procured from hydrochloric acid, whilst sodium carbonate, caustic soda, metallic sodium itself and all its compounds, may be obtained from sodium sulphate.

Even in the animal organism salt undergoes similar changes, furnishing the sodium, alkali, and hydrochloric acid which take part in the processes of animal life.

Its necessity as a constituent in the food both of human beings and of animals becomes evident when we consider that both hydrochloric acid and salts of sodium are found in the substances which are separated out from the blood into the stomach and intestines. Sodium salts are found in the blood and in the bile which is elaborated in the liver and acts on the food in the alimentary canal, whilst hydrochloric acid is found in the acid juices of the stomach. Chlorides of the metals are always found in considerable quantities in the urine, and if they are excreted they must be replenished in the organism; and for the replenishment of the loss, substances containing chlorine compounds must be taken in food. Not only do animals consume those small amounts of sodium chloride which are found in drinking water or in plants or other animals, but experience has shown that many wild animals travel long distances in search of salt springs, and that domestic animals which in their natural condition do not require salt, willingly take it, and that the functions of their organisms become much more regular from their doing so.

_The action of sulphuric acid on sodium chloride._--If sulphuric acid be poured over common salt, then even at the ordinary temperature, as Glauber observed, an odorous gas, hydrochloric acid, is evolved. The reaction which takes place consists in the sodium of the salt and the hydrogen of the sulphuric acid changing places.

NaCl + H_{2}SO_{4} = HCl + NaHSO_{4}
Sodium Sulphuric Hydrochloric Acid sodium
chloride acid acid sulphate

At the ordinary temperature this reaction is not complete, but soon ceases. When the mixture is heated, the decomposition proceeds until, if there be sufficient salt present, all the sulphuric acid taken is converted into acid sodium sulphate. Any excess of acid will remain unaltered. If 2 molecules of sodium chloride (117 parts) be taken per molecule of sulphuric acid (98 parts), then on heating the mixture to a moderate temperature only one-half (58·5) of the salt will suffer change. Complete decomposition, after which neither hydrogen nor chlorine is left in the residue, proceeds (when 117 parts of table salt are taken per 98 parts of sulphuric acid) _at a red heat only_. Then--

2NaCl + H_{2}SO_{4} = 2HCl + Na_{2}SO_{4}
Table Sulphuric Hydrochloric Sodium
salt acid acid sulphate

This double decomposition is the result of the action of the acid salt, NaHSO_{4}, first formed, on sodium chloride, for the acid salt, since it contains hydrogen, itself acts like an acid, NaCl + NaHSO_{4} = HCl + Na_{2}SO_{4}. By adding this equation to the first we obtain the second, which expresses the ultimate reaction. Hence in the above reaction, non-volatile or sparingly volatile table salt and sparingly volatile sulphuric acid are taken, and as the result of their reaction, after the hydrogen and sodium have exchanged places, there is obtained non-volatile sodium sulphate and gaseous hydrochloric acid. The fact of the latter being a gaseous substance forms the main reason for the reaction proceeding to the very end. The mechanism of this kind of double decomposition, and the cause of the course of the reaction, are exactly the same as those we saw in the decomposition of nitre (Chapter VI.) by the action of sulphuric acid. The sulphuric acid in each case displaces the other, volatile, acid.

Not only in these two instances, but in every instance, if a volatile acid can be formed by the substitution of the hydrogen of sulphuric acid for a metal, then this volatile acid will be formed. From this it may be concluded that the volatility of the acid should be considered as the cause of the progress of the reaction; and indeed if the acid be soluble but not volatile, or if the reaction take place in an enclosed space where the resulting acid cannot volatilise, or at the ordinary temperature when it does not pass into the state of elastic vapour--then the decomposition does not proceed to the end, but only up to a certain limit. In this respect the explanations given at the beginning of this century by the French chemist Berthollet in his work 'Essai de Statique Chimique' are very important. _The doctrine of Berthollet_ starts from the supposition that the chemical reaction of substances is determined not only by the degrees of affinity between the different parts, but also by the relative masses of the reacting substances and by those physical conditions under which the reaction takes place. Two substances containing the elements MX and NY, being brought into contact with each other, form by double decomposition the compounds MY and NX; but the formation of these two new compounds will not proceed to the end unless one of them is removed from the sphere of action. But it can only be removed if it possesses different physical properties from those of the other substances which are present with it. Either it must be a gas while the others are liquid or solid, or an insoluble solid while the others are liquid or soluble. The relative amounts of the resultant substances, if nothing separates out from their intermixture, depend only on the relative quantities of the substances MX and NY, and upon the degrees of attraction existing between the elements M, N, X, and Y; but however great their mass may be, and however considerable the attractions, still in any case if nothing separates out from the sphere of action the decomposition will presently cease, a state of equilibrium will be established, and instead of two there will remain four substances in the mass: namely, a portion of the original bodies MX and NY, and a certain quantity of the newly formed substances MY and NX, if it be assumed that neither MN or XY nor any other substances are produced, and this may for the present[24] be admitted in the case of the double decomposition of salts in which M and X are metals and X and Y haloids. As the ordinary double decomposition here consists merely in the exchange of metals, the above simplification is applicable. The sum total of existing data concerning the double decomposition of salts leads to the conclusion that from salts MX + NY there always arises a certain quantity of NX and MY, as should be the case according to Berthollet's doctrine. A portion of the historical data concerning this subject will be afterwards mentioned, but we will at once proceed to point out the observations made by Spring (1888) which show that _even in a solid state_ salts are subject to a similar interchange of metals if in a condition of sufficiently close contact (it requires time, a finely divided state, and intimate mixture). Spring took two non-hygroscopic salts, potassium nitrate, KNO_{3}, and well-dried sodium acetate, C_{2}H_{3}NaO_{2}, and left a mixture of their powders for several months in a desiccator. An interchange of metals took place, as was seen from the fact that the resultant mass rapidly attracted the moisture of the air, owing to the formation of sodium nitrate, NaNO_{3}, and potassium acetate, C_{2}H_{3}KO_{2}, both of which are highly hygroscopic.[24 bis]