and Attached Water. 39 



where mixtures of salts were employed, we find that while 

 with such mixtures the more powerful cryogen governs the 

 temperature, here it is the less powerful cryogen which does so. 

 § 174. Ferroeyanide of Potassium — The cryohydrate of this 

 salt forms at — l c, 7. Two analyses of the same residual liquid 

 were made by evaporating at 100°, and subsequent stirring 

 with free exposure to the air at the same temperature. 



grams. K 4 FeCy 6 . Per cent. 



6*6510 gave 0*7987 or 12 

 6-5080 „ 0-7665 „ 11-8 



The mean indicates the formula 



K,FeCy 6 + 151-6 H 2 0. 



The temperature of the cryogen is — 1° # 61. If this body had 

 been the first to be experimented on in the direction which 

 has led to the discovery of the cryohydrates, it is possible that 

 the whole class might have escaped notice, because, when the 

 cryohydrate is allowed to melt, a considerable quantity of yel- 

 low salt (terhydrate or subcryohydrate) falls down unless the 

 liquid is kept well agitated. Though not peculiar to this salt, 

 it is perhaps more marked with it than with any other. To 

 be viewed almost as a corollary to the above fact is the follow- 

 ing one concerning the same salt. When a solution of the 

 strength of the cryohydrate is cooled, there may be a simul- 

 taneous formation of ice and some hydrated or anhydrous 

 ferroeyanide : the one floats ; the other sinks. As before 

 pointed out in other cases, this independent separation of the 

 constituents of the cryohydrate is entirely prevented by pla- 

 cing in the solution a fragment of previously formed cryohy- 

 drate. 



§ 175. Ferricyanide of Potassium. — At a temperature of 

 — 3°*9 a saturated solution of this salt began to give up its 

 cryohydrate, having previously given up a less hydrated salt. 

 Though several determinations of the amount of salt in the 

 residual liquid were made both by evaporation over a water- 

 bath and drying in vacuo over sulphuric acid, the results dif- 

 fered from one another by several per cent. The lowest was 

 19*8 per cent., the highest 24 per cent. The temperature of 

 the cryogen is — 3 C '9 C. The ambiguity in the amount of 

 water in the cryohydrate finds its counterpart in the similar 

 ambiguity in the water of crystallization of the ordinary salt. 



§ 176. Nitrate of Urea. — The cryohydrate first appears at 

 —4°, on cooling a saturated solution. The residual liquids 

 from two preparations, after about the same quantity of cryo- 

 hydrate had separated, were very gently evaporated on a water- 



