ON WATKK AND ITS COMPOUND- 97 



Solutions of salts on refrigeration below deposit ice or crys- 

 tals (\vhich then usually contain water of crystallisation) of the salt 

 dissolved, and on arriving by this means at a certain degree of con- 

 centration they solidify in their entire mass. These solidified masses 

 are termed r>7/o// //'//<//' '*. My researches on solutions of common salt 

 (1868) showed that its solution solidifies when it reaches a composition 

 NaCl + 10H 2 O (180 parts of water per 58'5 parts of salt), which takes 

 place at about 23. The solidified solution melts at the same temper- 

 ature, and both the portion melted and the remainder preserve the 

 above composition. Guthrie (1874-1876) obtained the cryohydrates of 

 many salts, and he showed that certain of them are formed at com- 

 paratively low temperatures, whilst others (for instance, corrosive 

 sublimate, alums, potassium chlorate, and various colloids) are formed 

 on a slight cooling, to 2 or even before, and that these contain a 

 very large amount of water. One can easily imagine that these two 

 series of cryohydrates differ considerably from each other, but the in- 

 sufficiency of the existing data 58 does not permit of a true judgment 

 being formed. Nevertheless, in the case of common salt, the cryo- 



tating salts of these metals by hydrogen sulphide), and be then carefully washed (by 

 allowing the precipitate to settle, pouring off the liquid, and again adding sulphuretted 

 hydrogen water), then, as was shown by Schulze, Spring, Prost, and others, the pre- 

 viously insoluble sulphides pass into transparent (for mercury, lead, and silver, reddish 

 brown ; for copper and iron, greenish brown ; for cadmium and indium, yellow ; and for 

 zinc, colourless) solutions, which may be preserved (the weaker they are the longer they 

 keep) and even boiled, but which, nevertheless, in time become curdled that is, settle 

 in an insoluble form, and then sometimes become crystalline and quite incapable of 

 re-dissolving. Graham and others observed the power shown by colloids (see note 18) of 

 forming similar hydrusols or solutions of gelatinous colloids, and, in describing alumina, 

 and silica, we shall have occasion to speak of such solutions once more. 



In the existing state of our knowledge concerning solution, such solutions may be 

 looked on as a transition between emulsion and ordinary solutions, but no fundamental 

 judgment can be formed about them until a study has been made of their relations to 

 ordinary solutions (the solutions of even soluble colloids freeze immediately on cooling 

 below 0, and, according to Guthrie, do not form cryohydrates), and to supersaturated 

 solutions, with which they have certain points in common. 



58 Offer (1880) concludes, from his researches on cryohydrates, that they are simple 

 mixtures of ice and salts, having a constant melting point, just as there are alloys having a 

 constant point of fusion, arid solutions of liquids with a constant boiling point (see note 60). 

 This does not, however, explain in what form a salt is contained, for instance, in the 

 cryohydrate, NaCl + 10H 2 O. At temperatures above 10 common salt separates out in 

 anhydrous crystals, and at temperatures near 10, in combination with water of 

 crystallisation, NaCl + 2H 2 O, and, therefore, it is very improbable that at still lower 

 temperatures it would separate without water. If the possibility of the solidified cryo- 

 hydrate containing XaCl + 2H 2 O and ice be admitted, then it is not clear why one of 

 these substances does not melt before the other. If alcohol does not extract water from 

 the solid mass, leaving the salt behind, this does not prove the presence of ice, because 

 alcohol also takes up water from the crystals of many hydrated substances (for instance, 

 from NaCl + 2H 2 O) at about their melting-points. Besides which, a simple observation 

 on the cryohydrate, NaCl + lOH.^O, shows that with the most careful cooling it does not 



VOL. I. H 



