Mr. F. Guthrie on Salt Solutions and Attached Water. 9 



liquor was poured off into a weighed flask and partially solidified, 

 and so on five times in succession. The solid residues were then 

 allowed to melt and get to the temperature of the air; they were 

 then evaporated in the usual way. The portion which remained 

 to the last was not frozen, in order to see whether its composi- 

 tion was the same as that of the parts previously removed by 

 solidification. 



Table VII. 



Percentage of salt in fractionally solidified NaCl brine below 21°. 



Temperature of solidification. per cent. 



-21° to -22°, contained 23- 7232 of NaCl. 



-22 „ 23-6581 



-22 „ 23-7262 



-23 „ 23-8201 



-23 „ 23-3431 



-23 „ 23*3478 



The nearest molecular relationship indicated by these num- 

 bers is 



2NaCl + 21H 2 0. 



The formula NaCl-f 9H 2 requires 26*5 per cent, of salt. 

 NaCl + 10H 2 O „ 24-5 

 NaCl + llH 2 „ 22-4 

 2NaCl + 2lH 2 „ 23-6 



§ 14. In these the salient point is the composition of the 

 final mother-liquor, which is essentially the same as that of the 

 successively separated solids. 



Accordingly a salt-ice freezing-mixture is just capable of im- 

 poverishing saturated brine by withdrawal of salt-rich ingre- 

 dients (namely the bihydrate) to such an extent that the unso- 

 lidified part is homogeneous, in the sense of being solidifiable 

 as a whole. And such solidification takes place immediately 

 below the temperature —21° to —22°, which is the lowest 

 temperature to be got by an ice- salt freezing- mixture. I pre- 

 sume that if the two solids, ice and salt, could be presented to 

 one another in a state of indefinitely fine division, this propor- 

 tion of 23'6 of salt to 76*4 of ice would act most promptly and 

 continuously as a freezing-mixture, because the formation of the 

 bihydrate would not then occur. 



I am disposed to think that the hydrate of salt, the genesis 

 of which is here described, may have the composition 



iNaCl + 10H 2 O; 

 for under the circumstances of its formation and analysis I only 

 see one serious source of error ; and that is the condensation of 

 moisture from the air upon the surface of the cold brine. This 



