and Attached Water. 



453 



purified by several recrystallizations, was dissolved to satura- 

 tion at 12° C, and very gradually cooled to 0°. The crystals 

 given up in this range proved to be anhydrous iodide. On 

 further cooling to —12°, a second crop of crystals was formed, 

 which also proved anhydrous. Between —12° and —20° but 

 little further solidification occurred. When —23° was reached 

 rapid solidification began, and the temperature rose to —15°. 

 In another similar case the temperature sank to —24° before 

 solidification began. The whole becomes of a dough-like 

 pastiness. The crystals are long, silky needles ; they remain 

 transparent. On further cooling in a chloride-of-calcium 

 cryogen to —26°, the silvery opacity due to a true cryo- 

 hydrate is produced, and the whole becomes of a stony hard- 

 ness. It is clear, therefore, that with the iodide as with the 

 chloride of sodium there are two cryohydrates : — the subcryo- 

 hydrate, having a melting- and solidifying-point at — 15° ; and 

 another, which in combination with the first has a melting- 

 and solidifying-point at — 26°'5. We have therefore to look 

 for the true (that is, lowest-temp era tured) cryohydrate in 

 solutions weaker than that analyzed in § 65, which had 59*45 

 per cent. With the chloride of sodium there is no difficulty 

 in separating the subcryohydrate ; the latter body falls as 

 iridescent scales, and leaves the residual liquid clear. With 

 the iodide of sodium no such mechanical separation ensues, 

 and no artificial separation demanding the maintenance of so 

 low a temperature would be trustworthy. 



To get at the composition of the true cryohydrate it must 

 be approached from the other side — not from saturated, but 

 from dilute solutions (see § 123), I have accordingly exa- 

 mined in succession the temperatures at which ice or other 

 body separates from percentage solutions of Nal. 



Table XXVIII. 



Nal per cent., 

 by weight. 



H 2 O per cent., 



by weight. 



Temperature O. 

 at which solidifi- 

 cation begins. 



Nature of solid 

 separated. 



5 



95 



- 0-7 



Ice. 



10 



90 



- 21 



»» 



15 



85 



- 3-9 



»» 



20 



80 



- 60 





25 



75 



- 8-5 



>> 



30 



70 



— 11-8 



J> 



35 



65 



-15 2 



>> 



40 



60 



-20-5 



»J 



45 



55 



-260 



>> 



49-2 



50-8 



-300 



Cryohydrate. 



50 



55 



50 

 45 



-29-5 | 

 -20-0 f 



Subcryohydrate. 



60 



40 



-14-7 



>> 



616 



38-4 



00 



Nal. 



63-6 



36-4 



+ 13 



" 



