260 PHYSIOLOGICAL PHYSICS. [Chap. xxn. 



proportion of chloride of sodium in the various strata 

 ascertained. The results were very similar to those 

 obtained in diffusing the same salt in a jar of pure 

 water." * 



11 Diffusion of a crystalloid thus appears to proceed 

 through a firm jelly with little or no abatement of 

 velocity. With a coloured crystalloid, such as bi- 

 chromate of potash, the gradual elevation of the salt 

 to the top of the jar is beautifully illustrated. On 

 the other hand, the diffusion of a coloured colloid, 

 such as caramel, through the jelly appeared scarcely 

 to have begun after eight days had elapsed." 



IMaSysis. Graham thus perceived a method for 

 effecting separation by means of colloidal matter. To 

 this method he applied the term dialysis, and the ap- 

 paratus used he called a dialyser. This is made by 

 using vegetable parchment paper, which is unsized 

 paper altered by a short immersion in sulphuric acid 

 or in chloride of zinc. When wetted, the parchment 

 expands and becomes translucent. A piece of such 

 paper, wetted., is applied to a light hoop of gutta- 

 percha, two inches in depth and eight to ten inches in 

 diameter, so as to form a sieve. The paper ought to 

 rise up round the hoop, to which it is then firmly 

 secured by tying. Better still, a second hoop of 

 slightly greater diameter may be slipped up from 

 below, over the turned-up edge of parchment paper, 

 which it binds like a ring to the inner hoop. The 

 dialyser so prepared is seen to be sound by sponging 

 its inner side with water and finding that no wet 

 spots appear on the other side. If it be defective, 

 the defects are remedied by painting over with liquid 

 albumen, which is coagulated by holding over steam. 



The solution to be dialysed is poured into the hoop 

 to a depth of not more than half an inch, and the 



* Graham : "Liquid Diffusion Applied to Analysis;" Philos. 

 Trans. 1861, p. 199. 



