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liquor was poured off into a weighed flask and partially solidified, and so on five times in succession. The solid residues were then allowed to melt and get to the temperature of the air; they were then evaporated in the usual way. The portion which remained to the last was not frozen, in order to see whether its composition was the same as that of the parts previously removed by solidification.

TABLE VII.
Percentage of salt in fractionally solidified NaCl brine below 21°.
Temperature of solidification.

per cent.
-21° to -22°, contained 237232 of NaCl.
-22

23.6581
-22

23.7262 -23

23.8201 - 23

23-3431 -23

23.3478 The nearest molecular relationship indicated by these numbers is

2 NaCl +21 H,0. The formula NaCl + 9H, O requires 26-5 per cent. of salt. NaCl + 10H2O

24.5 NaCl +111,0

22:4 2NaCl +21H20

23.6 § 14. In these the salient point is the composition of the final mother-liquor, which is essentially the same as that of the successively separated solids.

Accordingly a salt-ice freezing-mixture is just capable of impoverishing saturated brine by withdrawal of salt-rich ingredients (namely the bihydrate) to such an extent that the unsolidified part is homogeneous, in the sense of being solidifiable as a whole.

And such solidification takes place immediately below the temperature -21° to -22°, which is the lowest temperature to be got by an ice-salt freezing-mixture. I presume that if the two solids, ice and salt, could be presented to one another in a state of indefinitely fine division, this proportion of 23.6 of salt to 76.4 of ice would act most promptly and continuously as a freezing-mixture, because the formation of the bihydrate would not then occur.

I am disposed to think that the hydrate of salt, the genesis of which is here described, may have the composition

NaCl +101,0; for under the circumstances of its formation and analysis I only see one serious source of error; and that is the condensation of moisture from the air upon the surface of the cold brine. This

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would tend to make the amount of salt found too small. To this point we shall bave to return.

§ 15. Physical Condition of a Freezing-mixture.— The fact that the rationale of a freezing-mixture is the liquefaction of solids seems to demand at once the conclusion that a freezing-mixture (of say ice and salt) must always be partially liquid.

Nevertheless, if a mixture be made of about three parts by weight of finely powdered rock-salt and one part of finely crushed ice, the liquefaction which ensues is followed by an apparent regelation so complete that the whole can be handled as a solid mass, and becomes indeed perfectly dry. The cause of this is, I have no doubt, due to the temporary existence of a supersaturated solution of NaCl + 2 H, 0, a body which is formed at about – 10°, Löwitz (-5°, Nölle) (from — 3° to -20° continuously, F. G.), when saturated brine is artificially cooled. It is, of course, impracticable to separate this substance from the general mass of the freezing-mixture with sufficient precision to allow of its analysis. The phenomenon of solidification, moreover, only lasts a few minutes. Liquefaction ensues, and the temperature, which had made a pause, again sinks rapidly.

$ 16. The minimum temperature of a salt-ice freezing-mixture seems to be attained between the somewhat wide margins of 3 of salt to l of ice, and 1 of salt to 2 of ice. The lowest temperature appears to be -21° to - 22° C.

§ 17. Liquid portion of Freezing-mixture.—It is clear that the liquid portion of a freezing-mixture is a brine of such a composition as to resist solidification at the temperature of the freezing-mixture. Accordingly we ought to find that the liquid portion of a freezing-mixture has the same percentage composition as the mother-liquor of a saturated brine from which the bihydrate has been separated out by the external application of an ice-salt freezing-mixture. To test how far this is verified by experience, dry ice and salt were mingled in three proportions, namely 3 of ice to l of salt, 1 of ice to 1 of salt, and I of ice to 3 of salt by weight. The salt and ice were in the finest state of division, and the ice was uniformly and as thoroughly dry as possible. The mixtures were constantly stirred in a wooden bowl for ten minutes, and then thrown upon flannel and pressed through. Ice. Salt.

NaCl. 3 1 gave a liquid containing 22.508 1 1

24:343 1 3

24.711

The amount of salt found is therefore not far off that contained in the 10 hydratc, namely 21.5.

Cryohydrates. § 18. General.—The discovery of the hydrate of chloride of sodium which contains about ten molecules of water to one of the anhydrous salt, caused me to look for similar combinations of water with other salts. This was of course all the more necessary, since if such combinations existed with the salts occurring in the sea, and if such combinations had solidifying-points within the range of the atmospheric temperature, the composition of the solid formed when sea-water freezes would be partly that of ice and partly that of the solid hydrates formed from the brine which had been enriched by the removal of water as ice.

§ 19. It has long been known that the presence of a soluble salt in water depresses the point at which the liquid solidifies (irrespective of the nature of the body separated by solidification). Suppose now we take a solution of the salt a b saturated at the ordinary temperature and cool it. We may, above 0° C., eitber get a separation of the anhydrous ab, or some crystalline combination of the salt with water, a hydrate. To this Na Cl is the only exception; for this body is equally soluble in water at all temperatures above 0o C. Putting Na Cl on one side, we may admit, then, that the mother-liquor gets poorer and poorer as the temperature falls. This is obviously the case if the anhydrous salt falls down ; and experience informs us that in order to dissolve a hydrated salt, water, and not the anhydrous salt, must be added. Experience also teaches that there is no marked discontinuity at 0° C. We never find the whole of the salt separated before or even at 0° C. One patent consequence of this is that we can form a freezing-mixture (whereof a portion is liquid) by mixing any soluble salt with ice. We get, therefore, in all cases a solution of the salt below 0° C. If as the temperature is still lowered anhydrous salt were to separate out, we should at last get pure water unfrozen below 0° C.-an impossible result. The same would ensue if a hydrate richer in salt than is the solution were to separate out; whereas if ice or a hydrate poorer in salt than is the solution were to separate out, we should then get at a lower and lower teniperature a richer and richer solution, and return to the very condition of strength which the lowering of temperature had altered. This is also an impossibility. Accordingly, in all cases some temperature below 0° C. must be reached at which, after separation of the anhydrous salt or a hydrate richer in salt than the solution, the water and salt solidify together, that is, in constant proportion. The effect of such solidification must be (1) the preservation of a constant temperature during solidification from the moment when the proper proportion between the water and the salt is reached, and (2) the preservation of that proportion exhibited by the identity of composition of any crop of the solid and the mother-liquor. It is clear that if ice and the hydrated or anbydrous salt separated ont in constant proportion and not combined, but merely mixed with one another, the mass would have a constant solidifying- and melting-point; and this would be below zero to the same amount as would be reached on mixing artificially the anhydrous or hydrated salt with ice in the same proportion. But when we have distinct and unchangeable relation by weight demanded by the constancy of the solidifying- and inelting-point, we have undoubtedly a numerical physical relation as fixed and no less important than the points of fusion or degrees of solubilities. And if, as I shall show, all the hydrates formed under these conditions have distinct crystalline forms, we have all the conditions of chemical association; at least I know of none other. It is an essential element in the existence of these compounds that they can only exist in the solid state below 0° C. Hence I propose to call them for the present “cryohydrates.” At the ordinary temperatures they melt in their own water of crystallization, and appear as ordinary solutions never saturated. And when once the proportion between the salt and the water in a cryohydrate has been found, the cryohydrate can be formed in any quantity by dissolving the salt in water in the required proportion. Such a solution shows no sign of yielding up ice or anhydrous salt (or other hydrate) until its temperature, ou being lowered, reaches a certain temperature peculiar to the salt (unless under supersaturation); it then solidifies as a whole, maintaining throughout that constant temperature. Above this temperature (that is, in the melted state) it is precisely in the same predicament as a salt melted in its own water of crystallization.

§ 20. The same cryohydrates are (and indeed must inevitably be) formed if we cool such a solution of the respective salt. in water as contains a greater proportion of water than the cryohydrate. As we have seen in §§ 10, 13, where the experiments dealt with brine of NaCl, ice is then formed, and the liquid gets richer and richer in salt, falling in temperature till the ratio proper to the cryohydrate is reached; whereupon, as before, homogeneous solidification ensues without abasement of temperature, If we approach the cryohydrates from this side (that is, by removing ice from a dilute solution), we are sure not to run the risk of being inconvenienced by the intervention of any interinediate hydrate similar to the bihydrate of chloride of sodium ($ 11). But such intermediate hydrates are of the rarest possible occurrence; so that, on account of the high specific and latent heat of water, it is invariably most convenient to start from a saturated solution.

preservation of that proportion exhibited position of any crop of the solid and the clear that if ice and the hydrated or anbyint in constant proportion and not comixed with one another, the mass would fying- and melting-point; and this would ame amount as would be reached on mixhydrous or hydrated salt with ice in the ut when we have distinct and unchangeat demanded by the constancy of the soli-point, we have undoubtedly a numerical xed and no less important than the points f solubilities. And if, as I shall show, all under these conditions have distinct cryse all the conditions of chemical associaw of none other. It is an essential elee of these compounds that they can only ate below 0° C. Hence I propose to call

cryohydrates.” At the ordinary tempei their own water of crystallization, and olutions never saturated. And when once en the salt and the water in a cryohydrate Cryohydrate can be formed in any quantity

in water in the required proportion. Such ign of yielding up ice or anhydrous salt (or

$21. Cryohydrate of Chloride of Ammonium.-A saturated solation of NH, Cl was cooled in ice and the liquid portion transferred to a beaker surrounded by an ice-salt freezingmixture. The temperature fell continuously, and anhydrous chloride of ammonium kept falling down. At -15° the solidifying part presented a different appearance. It then took the form of a brilliant white apparently flocculent mass lighter than the unsolidified liquid. After standing, with stirring, for a quarter of an hour, the temperature was still - 15o. The clear portion was poured off into a fresh beaker, to which the cold was applied. The solidifying parts are now seen to be minute crystals, very much resembling ice-flowers, but opaque. The sides of the beaker become studded with transparent crysstals of four sides, which are striated parallel to the sides. By and by these crystals become perfectly white and opaque, and a third axis of crystallization is developed, which was at first suppressed. The crystals are perfectly beautiful, resembling, where opaque, frosted silver, On allowing a thick cup to freeze and breaking it, an exquisite pearly appearance is presented. The structure appears then quite fibrous, the fibres running perpendicular to the axis of the cup; and the appearance, as far as structure is concerned, is similar to that of sublimed chloride of ammonium. The temperature remains constant at -15° C., even to perfect dryness. The first crops of crystals were rejected as being possibly contaminated with NH, Cl. The last crop and the mother-liquor were analyzed by being weighed into glass basins and evaporated at 100° C.

grms.

NH, I,
I.

9:5360 gave 1.7573, or 18.43
M.L. II. .

6.0890 1.1895 19:56 The analysis II., which is of the mother-liquor, corresponds nearly with the molecular relation NH, CI+ 12 H, O, which requires 19.98 per cent. of NHAC).

§ 22. Cryohydrate of Sulphate of Zinc.-The hydrates of sulphate of zinc already known are very numerous. They are :

Zn S04+ H2O,
ZnSO4+2 H20,
2Zn SO, +7 H,O,
Zn SO2 +51,0,
Zn SO4 +6H20,

Zn S04+7H20. The last, which is the ordinary form of zinc-vitriol, when saturating water at 17:5° C., gives a solution, according to Karsten, consisting of 52 per cent. of salt and 48 per cent. of water. On cooling such a solution to and below 0C., the heptahydrate

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ts temperature, ou being lowered, reaches
peculiar to the salt (unless under supersa-
difies as a whole, maintaining throughout
ure. Above this temperature (that is, in

precisely in the same predicament as a vater of crystallization. wohydrates are and indeed must inevicool such a solution of the respective salt greater proportion of water than the cryoen in $g 10, 13, where the experiments 1, ice is then formed, and the liquid gets It, falling in temperature till the ratio ate is rcached; whereupon, as before, on ensues without abasement of tempethe cryohydrates from this side (that a dilute solution), we are sure not to ouvenienced by the intervention of any pilar to the bihydrate of chloride of ch intermediate bydrates are of the

so that, on account of the high speer, it is invariably most convenient to

tion.

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