chap, xxii.j CRYSTALLOIDS AND COLLOIDS. 259 



character of their hydrates, their inertness in ordinary 

 chemical relations, and their mutability. To these he 

 applied the term COLLOID. 



One remarkable peculiarity of colloidal substances 

 is, that while themselves of extremely low diffusive 

 power, they afford a medium of diffusion. They 

 permit the highly diffusive substances to permeate 

 them readily, resist less diffusive substances, and en- 

 tirely cut off substances like themselves. Thus, a sheet 

 of very thin letter-paper, sized with starch (a colloid) 

 was formed into a tray and laid on the surface of 

 water. Into it a solution of cane sugar and gum 

 arabic was placed. In twenty-four hours three-fourths 

 of the whole sugar had passed through, while barely a 

 trace of the gum could be detected in the water. Of 

 colloidal substances, gum, albumin, gela-tin, and starch 

 are the chief examples. 



A very interesting experiment, described by 

 Graham, shows how colloidal substances in mass are 

 nearly as good media for diffusion as water. " Ten 

 grammes of chloride of sodium, and two grammes of 

 Japanese gelatine, or gelose of Pageii, were dissolved 

 together in so much hot water as to form 100 cubic 

 centimetres of fluid. Introduced into an empty 

 diffusion jar, and allowed to cool, this fluid set 

 into a firm jelly, occupying the lower part of the jar, 

 and containing of course 10 per cent, of chloride of 

 sodium. Instead of placing pure water over this jelly, 

 it was covered by 700 cubic centimetres of a solution 

 containing 2 per cent, of the same gelose, cooled so far 

 as to be on the point of gelatinising, the jar at the 

 same time being placed in a cooling mixture in order 

 to expedite that change. The jar, with its contents, 

 was now left undisturbed for eight clays at a tempera- 

 ture of 10. After the lapse of this time, the jelly 

 was removed from the jar in successive portions of 

 50 cubic centimetres each from the top, and the 



