and Attached Water. 



217 



Since it will be shown in § 161 et seq. that the colloid forms 

 of matter not only do not unite with water, but also do not 

 depress the freezing-point of that liquid, it did not seem un- 

 likely that on cooling below 0° a solution of raw sugar con- 

 taining about 50 per cent, of solid matter, the solid cryohy- 

 drate would be formed and the sugar be freed from colloid 

 impurities. As caramel certainly, and probably other foreign 

 colouring-matters in raw sugar are colloids, I have succeeded 

 in greatly purifying raw sugar by this method of cryohydra- 

 tion. Into the details of this I shall not here enter. 



§ 159. Glycerine. — With regard to this substance a very 

 remarkable circumstance may be noticed. That it is crystal- 

 loid we have had until lately (1) the indirect evidence de- 

 pending upon its being an alcohol, and upon several alcohols 

 being known in the solid and crystalline state, while others 

 which are not so known yet unite with crystalline salts ; (2) 

 the direct evidence obtained from its diffusion through colloid 

 septa. Lately it has been observed to assume the form of a 

 crystalline solid. Again, it has lately been employed in aqueous 

 solution in Pictet's ice-machine as a non-freezable liquid to 

 yield heat to vaporizing sulphurous acid, and take it from 

 water for the purpose of freezing the latter. The latter faculty 

 of its solution to resist solidification below 0° 0. proves, first, 

 that it will form a cryogen, and, secondly, that it will form a 

 cryohydrate — the latter fact again proving, as we shall see in 

 § 161 et seq., that it is a crystalloid. 



Pure glycerine dried by being kept for a week over oil of 

 vitriol in vacuo, when mixed with finely crushed ice forms a 

 cryogen whose temperature is —19° C. 



Table XXXVIII. 



C 3 H 8 3 per 

 cent, by weight. 



H 2 O per cent, 

 by weight. 



Temperature 

 at which solidi- 

 fication begins. 



Nature of solid 

 formed. 



5 



95 



o 

 - 0-8 



Ice. 



10 



90 



- 2 





15 



85 



- 33 





20 



80 



- 5 





25 



75 



- 6-2 





30 



70 



- 8-8 





35 



65 



-115 





40 



50 



— 13*9 





45 



55 



-167 





I have not yet succeeded in obtaining the cryohydrate of 

 glycerine. 



