208 Mr. T. Graham on Liquid Diffusion applied to Analysis, 



required for diffusion, and makes its way through the gelatinous 

 septum. Gum, on the other hand, possessing as a colloid an 

 affinity for water of the most feeble description, is unable to 

 separate that liquid from the gelatinous starch, and so fails to 

 open the door for its own passage outwards by diffusion. 



The separation described is somewhat analogous to that ob- 

 served in a soap-bubble inflated with a gaseous mixture composed 

 of carbonic acid and hydrogen. Neither gas, as such, can pene- 

 trate the water-film. But the carbonic acid, being soluble in 

 water, is condensed and dissolved by the water-film, and so is 

 enabled to pass outwards and reach the atmosphere ; while 

 hydrogen, being insoluble in water, or nearly so, is retained be- 

 hind within the vesicle. 



It may perhaps be allowed to me to apply the convenient term 

 dialysis to the method of separation by diffusion through a 

 septum of gelatinous matter. The most suitable of all substances 

 for the dialytic septum appears to be the commercial material 

 known as vegetable parchment or parchment-paper, which was 

 first produced by M. Gaine, and is now successfully manufactured 

 by Messrs. De la Rue. This is unsized paper, altered by a short 

 immersion in sulphuric acid, or in chloride of zinc, as proposed 

 by Mr. T. Taylor. Paper so metamorphosed acquires consider- 

 able tenacity, as is well known ; and when wetted it expands and 

 becomes translucent, evidently admitting of hydration. A slip 

 of 25 inches in length was elongated 1 inch in pure water, and 

 1'2 inch in water containing one per cent, of carbonate of potash. 

 In the wetted state parchment-paper can easily be applied to a 

 light hoop of wood, or, better, to a hoop made of sheet gutta 

 percha, 2 inches in depth and 



8 orlO inches in diameter, so Fi g- L— Hoop Dialyser. 



as to form a vessel like a sieve 

 in form (fig. 1). The disk of 

 parchment-paper used should 

 exceed in diameter the hoop to 

 be covered by 3 or 4 inches, 

 so as to rise well round the 

 hoop. It may be bound to 

 the hoop by string, or by an 

 elastic band, but should not 

 be firmly secured. The parch- 

 ment-paper must not be po- 

 rous. Its soundness will be 

 ascertained by sponging the 

 upper surface with pure water, and then observing that no wet 

 spots show themselves on the opposite side. Such defects may 

 be remedied by applying liquid albumen, and then coagulating 



