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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 out 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 melting-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, on 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 intermediate 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.

§ 21. Cryohydrate of Chloride of Ammonium.-A saturated solution of NH4Cl 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 -15°. 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.

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NH, Cl. per cent.
1.7573, or 18.43

1.1895 19.56

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The analysis II., which is of the mother-liquor, corresponds nearly with the molecular relation NH, C1+12 H2O, which requires 19.98 per cent. of NH, Cl.

4

§22. Cryohydrate of Sulphate of Zinc.-The hydrates of sulphate of zinc already known are very numerous. They are:Zn SO4+ H2O,

4

2

2

Zn SO +2 H2O,
2Zn SO+7H2O,
Zn SO4+5 H2O,
Zn SO+6H,O,

4

2

Zn SO4+7H2O.

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 0° C., the heptahydrate

crystallizes out; and this, consisting of 56 of salt to 44 of water, impoverishes the mother-liquor until the latter contains 30.84 per cent. of the anhydrous sulphate. The temperature is now -7° C., and it remains constant at this degree. The last fraction having solidified at this temperature, was remelted and the water estimated by evaporation and heating to 240° C.

8.1531 grms. contained 2.5146 grms. of Zn SO4,
or 30.84 per cent.

This corresponds very closely with the composition
Zn SO4 + 20 H2 0.

It is noteworthy that this cryohydrate, after standing some days in a hermetically sealed tube, deposits massive rhombic crystals and a fine powder. I have not analyzed these; they are possibly the monohydrate and one of the intermediate hydrates insoluble in the fused cryohydrate.

§ 23. Cryohydrate of Sulphate of Magnesium.-Combinations are known consisting of 1 molecule of sulphate of magnesium combined with 1, 2, 6, 7, and 12 molecules of water. The last is the more interesting because Fritsche describes its formation from a saturated solution of the sulphate when cooled below 0° C. I find that when a saturated solution is cooled to -5° C. and transferred to a clean vessel, it may, if perfectly free from crystals of the 7-hydrate, be cooled to -10° C. without any further solidification. As soon, however, as further cold causes crystallization, the temperature rises to -6° C., and remains. constant at this point during the whole of the subsequent solidification, provided that a crystal of the previous crop is put into the cooled mother-liquor after each decantation.

The composition was determined by heating to 240° C. 7.6564 grms. gave 1·6736 grm. of anhydrous sulphate, or 21.86 per cent.

This corresponds to 23-83 (say 24) molecules of water. Its molecular ratio seems therefore to be

Mg SO4 + 24H2O.

§ 24. Cryohydrate of Nitrate of Potassium.-As far as I can inform myself, nitre, like the chloride of ammonium, has not hitherto been combined with water. The solution saturated at 20° C. gives an abundant crop of nitre at 0° C. There appears to be no intermediate hydrate, the body which separates at -2°.5 C. being apparently pure nitre. At -27 another body is formed, whose crystalline form resembles ice. It adheres to the side of the vessel; when separated, it floats, being lighter than the mother-liquor at -207; but when thrown into water at 20° C., it sinks, showing that it is not ice. The last four crops

of crystals were analyzed. The final one was formed also exactly at -2°.7.

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KNO,. per cent.

0·9221 or 10-4

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0.1959 11.7

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The molecular relation is therefore about 1: 44-6.

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§ 25. Cryohydrate of Sulphate of Copper.-The ordinary hydrates of blue vitriol are the mono-, bi-, and pentahydrates. On cooling a saturated solution, the constant temperature of solidification is found to be -2° C. The last crop of crystals being remelted, was evaporated and heated to the anhydrous

state.

6.6952 grms. gave 1·1312 grm. CuSO4, or 16.89 per cent. This corresponds to the atomic relation of 1 : 437 (sayl to 44) Cu SO4+44 H, O.

Each crop of crystals, when melted, and the mother-liquor present identically the same depth of colour.

§ 26. Cryohydrate of Sulphate of Sodium.-A very great many ordinary hydrates of this salt are known. I find that a saturated solution has a solidifying-point at -0°.7 C.

4-0630 grms. contained 1.850 grm. of Na2 SO4,

or 4.55 per cent.,

corresponding to the molecular ratio of 1 : 165·6 (say 1 to 166), Na, SO4 +166H2 O.

§ 27. Cryohydrate of Chlorate of Potassium.-The chlorate of potassium, like the nitrate, has not hitherto been combined with water. Almost the whole of the salt separates out in the anhydrous state when a saturated solution is cooled to 0° C. Ón further cooling to -3° C., the solution may present a remarkable condition of double supersaturation. If at this temperature a crystal of anhydrous chlorate is dropped in, anhydrous chlorate is formed in considerable quantity. If an ice fragment is introduced, ice only is formed. If both are thrown in, both are formed, the one set of crystals floating, the other sinking. If the temperature of the supersaturated solution be further cooled, the proper cryohydrate separates out and the temperture rises to -0°5. The normal formation ensues on introducing a crystal of the cryohydrate from one crop to start the formation of the next. This phenomenon, which is not without its counterpart in some other cases, argues forcibly for the existence of a distinct crystalline form proper to the cryohydrate.

(1) 13-2512 grms. of cryohydrate gave 0·3912 grm. KCIO, or 2.95 per cent.

(2) 13.5810 grms. of cryohydrate gave 0.3915 grm. KCIO, or 2.88 per cent.

The molecular relation is exactly

KCIO2+222 H2 0.

2

§ 28. Cryohydrate of Bichromate of Potassium.-I chose this salt because it is not constructed on the ordinary salt type. I found it solidified entirely at -1° C. The crystals, which are at first transparent, become opaque, and are of a bright strawyellow. 5.4968 grms. gave 0.2868 K, Cr2O7, or 5·305 per cent. This gives the largest ratio of water, namely

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As with the sulphate of copper, so in this case all crops of crystals, when melted, had exactly the same depth of colour.

$29. Sources of error.-I need scarcely point out that the molecular ratios derived from the above experiments can only be considered provisional. In the case of bodies which combine with as many as forty and upwards molecules of water, a very slight error in the determination may make a difference of one or two molecules of water. In those which have a very low solidifying-point, another source of error creeps in. The liquids, even on pouring from one vessel to another, dilute themselves by condensing moisture from the air. With due regard to the probable direction of error in each case, I venture to submit the following tabulation, showing in column 1 the anhydrous salt, in 2 the temperature Centigrade of solidification, in 3 the actual percentage of anhydrous salt found, and in 4 the number of molecules of water to one of salt.

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