454 On Salt Solutions and Attached Water. 



With regard to the first eight determinations there is no- 

 thing particular to remark. For the 40-per-cent. solution a 

 CaCl 2 ciyogen had to be used. Finding that ice separated 

 from the 45 solution, and the subcryohydrate from the 50 

 solution, I concluded that the composition of the true cryo- 

 hydrate must lie between these values. I accordingly sub- 

 mitted several ounces of the 45, and also of the 50 solution to 

 a carbonic-acid cryogen. From the first ice separated ; from 

 the second the subcryohydrate. The temperature sank in 

 both cases to —30°, and there remained constant, while from 

 both crop after crop of the pearly-white true cryohydrate was 

 formed. After about three quarters of each had been thus 

 removed, the temperature still remaining at —30°, the residual 

 liquids were analyzed by evaporation and strong heating. 



Of 45 solution 10*9945 gave 5*3982 Nal or '49*1 per cent. 

 Of 60 „ 14*6176 „ 7*1932 „ ,,49*2 „ 



Accordingly the true cryohydrate is reached from either 

 side. The water worth of this salt is 8*603. It would only 

 be possible to deduce the composition of the subcryohydrate 

 if we knew the relative quantities of the two present, as well 

 as the composition of their compounds. The reason why the 

 dry iodide does not give the temperature of —30° when used 

 as a cryogen is no doubt because, like chloride of calcium, it 

 evolves a large amount of heat in coming into contact with 

 water. But in spite of this deficit, the temperature of the 

 cryogen is governed, as we have seen it to be before, by that 

 of its coldest constituent. The speculation concerning iodide 

 of sodium in § 69* is fully justified. 



For the solubility at 0°, a solution saturated at 8° was kept 

 for three hours at 0° C. Of this, 10*7932 grms. contained 

 6*6486 grms. of this salt: this gives 61*6 per cent.; Kremers 

 found 61*3. 



§ 147. No salt can well be more favourable than the iodide 

 of sodium for the study of double supersaturation. The values 

 of the solution between 49*2 and 61*6 show this phenomenon 

 in a remarkable manner. Thus a solution of 60 per cent, 

 may be kept for hours at —20°. It may be stirred, shaken, 

 particles of dust and particles 'of Nal may be thrown in, a 

 glass rod covered with a film of hoar-frost inserted, it remains 

 quite limpid. Cooled a few degrees lower, it becomes sud- 

 denly semisolid, and rises to —15° or —14° in temperature. 

 At values approaching the cryohydrate, but rather less strong, 

 it may happen, as the excess of water is removed by cold, 

 that instead of the cryohydrate being formed when the right 



* In this paragraph read " cryohydrate of iodide of sodium " instead 

 of " cryohydrate of sodium." 



