Dialysis. 383 



offer scarcely any impediment to the diffusion of a crystalloid 

 throughout its entire mass. This diffusion will also take place 

 through any soft solid with almost equal rapidity; a very 

 familiar example of this fact is shown in the process of salting 

 meat, in which case the rapidly diffusible crystallizable sea-salt 

 penetrates to the interior of the flesh, which is a combination 

 of different colloid bodies, such as fibrin, albumen, gelatine, etc. 



Upon the fact that crystalloid bodies possess the power of 

 diffusing themselves through soft solids depends the operation 

 known as dialysis, and the construction of the instrument called 

 the dialyser. This consists simply of a tambourine-shaped 

 frame of gutta-percha, over which is tightly stretched a piece 

 of parchment paper, which completes the resemblance to that 

 musical instrument. This parchment paper is quite impervious 

 to water, so that no passage of fluid similar to filtration can take 

 place through it. If the dialyser be floated on the surface of 

 pure water, and a mixed solution of a crystalloid and a colloid 

 body be poured into it, the process termed dialysis immediately 

 commences; all the crystalloid matter passes through the 

 parchment paper into the water, and the colloid matter remains 

 behind in the dialyser. As an instance of its action, let us 

 suppose a mixed solution of sugar and gum to be poured into 

 the dialyser, when the sugar passes through into the water 

 below, and the gum remains behind in a pure form. If a mix- 

 ture of the beautiful aniline dye known as magenta and some 

 burnt sugar or caramel be employed, the passage of the ma- 

 genta into the pure water is readily observed, the dark-brown 

 uncrystallizable colloid caramel remaining in the dialyser. 



Other facts of great interest have been discovered as the 

 results of these investigations. Thus it is found that by 

 means of dialysis, we may obtain pure in solution many sub- 

 stances hitherto regarded as being perfectly insoluble. Amongst 

 these may be mentioned silica, alumina, Prussian blue, peroxide 

 of iron, stannic acid, and numerous other bodies of a similar 

 character. 



For example, if a solution of soluble glass, which is formed 

 by fusing silica with an excess of soda, be taken and acidified 

 with hydrochloric acid, the acid unites with the soda, forming 

 common salt, or chloride of sodium, the silica remaining for 

 some time dissolved in a gelatinous or colloid form, mixed with 

 the solution of the chloride of sodium. If, however, this mix- 

 ture of gelatinous silica and common salt be placed in the 

 dialyser, the salt rapidly diffuses itself into the water in the 

 outer vessel, and the solution of pure silica in water remains 

 in the dialyser. This solution is found to have a feebly acid 

 reaction on test paper, but not to the taste, as, being a 

 colloid, it cannot pass through the membrane of the tongue 



