. 



PIFFUSIOX. 



all ordinary current*, and even to sustain a considerable difference of pressure 



on iu oppoaite sides. 



But then? is another class of cases in which a liquid or a gas can pass 

 through a diaphragm, which is not, in the ordinary sense, porous. For instance, 

 when carbonic acid gas is confined in a soap bubble it rapidly escapes. The 



i> absorbed at the inner surface of the bubble, and forms a solution of 



carbonic acid in water. This solution diffuses from the inner surface of the 

 babble, where it is strongest, to the outer surface, where it is in contact 

 with air, and the carbonic acid evaporates and diffuses out into the atmosphere. 

 It w also found that hydrogen and other gases can pass through a layer of 

 caoutchouc. Graham shewed that it is not through pores, in the ordinary 

 tftim, that the motion takes place, for the ratios are determined by the 

 chemical relations between the gases and the caoutchouc, or the liquid film. 



According to Graham's theory, the caoutchouc is a colloid substance, that 

 is, one which is capable of combining, in a temporary and very loose manner, 

 with indeterminate proportions of certain other substances, just as glue will 

 form a jelly with various proportions of water. Another class of substances, 

 which Graham called crystalloid, are distinguished from these by being always 

 f definite composition, and not admitting of these temporary associations. 

 When a colloid body has in different parts of its mass different proportions 

 i >f water, alcohol, or solutions of crystalloid bodies, diffusion takes place through 

 the colloid body, though no part of it can be shewn to be in the liquid state. 



On the other hand, a solution of a colloid substance is almost incapable 

 of diffusion through a porous solid, or another colloid body. Thus, if a solu- 

 tion of gum and salt in water is placed in contact with a solid jelly of gelatine 

 and alcohol, alcohol will be diffused into the gum, and salt and water will 

 be diffused into the gelatine, but the gum and the gelatine will not diffuse 

 into each other. 



There are certain metals whose relations to certain gases Graham explained 

 by this theory. For instance, hydrogen can be made to pass through iron and 

 palladium at a high temperature, and carbonic oxide can be made to pass 

 through iron. The gases form colloidal unions with the metals, and are diffused 

 through them as water is diffused through a jelly. Root has lately found 

 that hydrogen can pass through platinum, even at ordinary temperatures. 



By taking advantage of the different velocities with which different liquids 

 and gases pass through parchment-paper and other solid bodies, Graham was 



