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the brines are supersaturable in regard to ice, so that the temperatures are most exactly determined by two observations. A good deal of ice is allowed to form; and this is then suffered nearly entirely to disappear; the temperature is observed at which the ice begins to increase in quantity when the brine is again subjected to cold.
Temperatures at which Brines of various strengths give up Ice.
Again the brines which contain 10 to 11 per cent. of salt. have singular behaviour in regard to the temperature at which they yield ice. These are the very solutions, it will be remembered, which behave singularly in respect of their refractive indices and also of their boiling-points. On comparing with Table III., this singularity does not manifest itself in regard to the specific gravities.
§ 11. Effect of cooling saturated Brine.-It is seen from Table VI. that, as far as the strengths of brine there examined extend, the stronger the brine the greater the cold required to separate ice from it. The strongest brine there examined con-1 tains 18.389 per cent. of NaCl. If saturated brine (containing 26.27 per cent. of NaCl) be cooled, quite a different class of phenomena ensues. Down to 0° C. no solidification whatever ensues either of ice or of salt. At -7° crystals of the bihydrate are observed to fall (NaCl+2H2O). These present a beautiful appearance of iridescent scales heavier than the mother-brine. Their composition has been examined by Löwitz, Fuchs, Nölle,
and Mitscherlich, whose analyses, however, are not in good accord, that of Löwitz being as much as 10 per cent. different from the calculated percentage of the bihydrate. Ehrenberg and Frankenheim state that the same hydrate is produced when a dilute solution of chloride of sodium evaporates at 15° C., and that it suddenly converts itself into the cubical anhydrous salt and water. At -22° C. the whole of this hydrate appears to be removed. The gradual impoverishment of the mother-brine was tested by maintaining a brine, at first saturated, for half an hour at the successive temperatures -10°, -16°, and -21° to -22° C. The brine had been previously kept at 0° for an hour. The original brine contained 26-2724 of NaCl per cent. It was the mother-brine from each crop of crystals which was subjected to the lower temperature.
Saturated brine at 0° contained
per cent. 26.2724 of NaCl. 24.6528
It is clear, therefore, that down to -21° the solidification impoverishes the brine, a result which might be due to the formation of the bihydrate or the precipitation of the anhydrous salt, but which is inconsistent with the formation of ice alone. The determinations were made by weighing 10 or 12 grammes of the brines, when at the atmospheric temperature, into longnecked flasks, evaporating and heating to about 300° C.
§ 12. A quantity of brine which had been thus impoverished by being kept for an hour at -21° to -22° was decanted into another flask and further cooled by contact with solid carbonic acid and ether. The whole (3 or 4 ounces) solidified; and the temperature remained perfectly constant at -22° to -23° (say -23°) until the last drop had frozen at -23°; it then sank rapidly.
If the vessel be continually shaken during crystallization, the form of the crystals may be very clearly seen. While the anhydrous salt crystallizes in the well-known cubes, the bihydrate separates as iridescent scales, and the body we are now considering solidifies in acicular bundles radiating from nuclei, and much resembling in appearance the supersaturated solution of sulphate of sodium when solidifying. It is, however, of a more than pearly whiteness, and finally of complete opacity.
§ 13. To see whether we have here indeed a definite hydrate, the whole was remelted by the warmth of the hand and successively partially refrozen under continual agitation. Each mother
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.
Percentage of salt in fractionally solidified NaCl brine below 21°.
The nearest molecular relationship indicated by these numbers is
2 NaCl +21 H2O.
The formula NaCl + 9H, O requires 26.5 per cent. of salt.
NaCl +10H, O
2NaCl +21 H2
§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 + 10H2O;
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
would tend to make the amount of salt found too small. To this point we shall have 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+2H, O, 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 1 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 1 of salt, 1 of ice to 1 of salt, and 1 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.
The amount of salt found is therefore not far off that con
tained in the 10 hydrate, namely 24.5.
§ 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., either get a separation of the anhydrous a b, 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 0° 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 temperature 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