14 Mr. P. Guthrie on Salt Solutions and Attached Water, 



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 remeltedand the water 

 estimated by evaporation and heating to 240° C. 



8*1531 grms. contained 2*5146 grms. of Zn S0 4 , 

 or 30*84 per cent. 



This corresponds very closely with the composition 



ZnSO 4 + 20H 2 O. 



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 hy- 

 drates 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 soli- 

 dification, 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 



MgS0 4 + 24H 2 0. 



§ 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 — 2°*7 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 — 2°*7; but when thrown into water at 

 20° C, it sinks, showing that it is not ice. The last four crops 



