366 



Frederick Guthrie on Salt Solutions 



Table XXV. 



MgS0 4 



per cent, by 

 weight. 



H 2 



per cent, by 

 weight. 



Temperature 



T at, which 



solidification 



begins. 



Nature of 

 solid formed. 



5 



95 



o 

 - 0-6 



Ice. 



10 



90 



- 15 





15 



85 



- 30 





20 



80 



- 4-8 





21-86 



7814 



- 50 



Cry ohyd rate. 



21-9 



781 



00 



MgS0 4 + 7H 2 0. 



25 



75 



+ 150 





30 



70 



4-310 



>♦ 



As my determination of the solubility at 0° C. (21*9) differs 

 considerably from some of those given by others (Gay-Lussac 

 20-5, Graham-Otto 20*5, Pfaff 21-9), I determined for myself 

 the solubilities above 0°. 



§ 137. Separation of Ice or Cryohydrate or Anhydrous Ni- 

 trate of Silver from a solution of that salt. — I have not yet de- 

 scribed the temperature and composition of the cryohydrate 

 of this salt. A solution saturated at 0° C. and further cooled 

 gives rise to an abundant crop of iridescent scales, which on 

 examination prove to be the anhydrous nitrate. This continues 

 till — 6°*5, when the opaque cryohydrate is formed. The solu- 

 tion, however, is particularly prone to double supersaturation. 

 A solution may sometimes be cooled to — 8° ; and during this, 

 while the crystals of the nitrate fall, ice-crystals are formed 

 which float. By and by the true cryohydrate is formed on the 

 side of the glass ; and then both separate crystallizations cease. 

 This double ^supersaturation is entirely prevented by introdu- 

 cing a fragment of the previously formed cryohydrate into the 

 mother-liquor which is being further cooled. The temperature 

 remains then constant at — 6°'6 to the last drop. The final 

 crop being remelted gave, on analysis, the following results ; — 

 14*3538 grms. gave 6*9442 grms. of AgN0 3 , or 48*38 per cent, 

 of AgN0 3 in the cryohydrate. The cryohydrate has accordingly 

 the composition AgN0 3 + 10*09 H 2 O. The temperature — 6°*5 

 is obtained on employing nitrate of silver as a cryogen. There 

 are few more beautiful colourless bodies than the cryohydrate 

 of silver. Opaque, lustrous, and semimetallic, it is somewhat 

 heavier than the liquid in which it is formed. 



Of a solution saturated at 0° I found that 6*3581 grms. gave 

 3*5068 of the anhydrous nitrate, or 55*01 per cent. Kremers 

 found 54*9 ; and accordingly I have availed myself of his deter- 



