26 ACTION OF BINARY ARRANGEMENTS. 



experiment, that a solitary gas has a tendency to expand itself to a certain extent, hut 

 not farther ; and we are equally assured that hodies, whether of the same or of dif- 

 ferent kinds, have an inclination to penetrate into each other. Where there is an ap- 

 parent indisposition to do this, we are not without plausihle reasons for supposing it to 

 be through the intervention of disturbing causes. If oil and water do not commingle, it 

 is a result determined by the action of their cohesion, as compared with the force of 

 attraction between them. An interesting example of this nature is afforded by the ac- 

 tion of mercury on glass : under ordinary circumstances, they show no disposition to 

 unite, not even so much as water and oil ; but, by a suitable application of heat, the co- 

 hesion of the mercury may be so lessened, and its force of attraction for glass at the 

 same time so exalted, that it can be brought to wet it ; an experiment first successfully 

 performed by Laplace. 



74. This nisus, or endeavour of one body to diffuse itself into the interstices of an- 

 other, has, under a variety of forms, been long recognised. The solution of salts, the 

 absorption of gas by liquids, the passage of liquids through crystals, the permeation of 

 porous textures, the diffusion of gases, the languid movement occurring in solids, were 

 known long ago. Of late jears, some extension of these facts has been obtained, and 

 the new phenomenon, though explicable on the same principles, is dignified by the title 



ENDOSMOSIS (37). 



75. For the explanation of the whole of this most interesting series of results, one 

 postulate alone is demanded that all bodies liave a tendency to diffuse themselves into 

 the interstices of all others, with more or less intensity. Nor is it difficult to admit this 

 principle in its fullest extent, when we consider the numerous examples philosophy af- 

 fords of it. All kinds of chemical absorptions and solutions are cases of it. The dis- 

 turbing causes which sometimes change, or even entirely hinder these actions, we shall 

 consider hereafter. 



76. BINARY ARRANGEMENTS, or those in which two bodies are engaged, whether solid, 

 liquid, or gaseous, exhibit some circumstances which it is here necessary to point out. 

 Let us suppose the couple under consideration to be oxygen gas exposed to an equal 

 volume of water. No remarkable phenomena attend the passage of the gas into the 

 liquid ; there is no rise of temperature, and the whole amount absorbed is greatly less 

 than the bulk of the water. If another gas be substituted, as carbonic acid, though 

 much more soluble, there is still no indication of change of temperature, but ammonia 

 and muriatic acid condensing to a much greater amount, disengage heat. Another 

 couple might be assumed, as charcoal or porous masses, with oxygen or other gases, and 

 similar indications be obtained. 



77. If, after a liquid has absorbed as much of any given gas as it is capable, we re- 

 move the remnant of unabsorbed gas, and in its place substitute some other of a differ- 

 ent kind, complex reaction ensues. The gas, already absorbed by the water, has its 

 condition of equilibrium disturbed, and, in conformity with the general principle (75), it 

 has a tendency to diffuse itself out of the water into the newly-introduced gas. This, 

 in its turn, has also a tendency to pass into the water. Thus, if over a volume of water, 

 impregnated with carbonic acid, and confined in a jar over mercury, we place a vol- 



