24 DIFFERS FROM CHEMICAL AFFINITY. 



66. Let us extend our supposition by placing another volume in presence of the for- 

 mer, differing from it in chemical composition alone. That difference would determine 

 certain motions of penetration, in addition to those resulting from mere mechanical action. 

 Not only would the united mass move so as to assume a mechanical equilibrium, but 

 its constituent parts would also move, so as to establish a chemical equilibrium at the same 

 time. Wherever an atom of one of the vapours existed, there would be found one of 

 the other also. To bring about this result, a mutual penetration of parts is demanded, a 

 transit of the constituents of one vapour among those of the other. The motions that 

 effect this arrangement take place without any resistance, just in the way that the light 

 of a distant star comes into our system, undeterred by the rays of our sun, and moves 

 freely in every direction ; his beams also move in the vacuum, intersecting the paths of 

 other luminous bodies without any hinderance or shock. 



67. In this state of extension, when the component atoms of a gas or vapour are sup- 

 posed to be stretched to their utmost limit, which we are prone to imagine can only be 

 done by an increase of the distance usually existing between an atom and its neigh- 

 bours, it is not difficult to suppose that these different motions can go on, and that a 

 foreign atom may insinuate itself in the interstices between others. But our ideas of 

 space and size being only relative, and as we know nothing of the dimensions of an ulti- 

 mate atom, nor of the interval that parts it from those around it, it is plain we could not, 

 without actual experiment, determine when a body had arrived at that state of con- 

 densation, or when its particles had become so closely approximated to each other as to 

 refuse the admission of foreign atoms between them. 



68. A mass of any kind in a vacuum, and undisturbed, moves, therefore, only in that 

 manner which the laws of dynamics indicate. Motions of another kind, however, are 

 induced when the vacuum is changed for a substance ; a kind of penetration, permeation, 

 or absorption is the result ; nor do the mechanical conditions of bodies appear to have 

 any effect : with some of these phenomena we are familiar. A gas, a liquid, or a solid 

 may indiscriminately pass by solution into the pores of water without any reference 

 to their aggregation. A variety of words have been used to express this action : solution, 

 endosmosis, permeation, &c. ; but, parting from the simplest experimental condition, we 

 shall have occasion to see that all these refer to varieties of one phenomenon only. 



69. If a solitary body has thus no opportunity of exhibiting the conditions of its 

 own arrangement as to structure or the forces that inhabit the interstices of its atoms, it is 

 very different with a binary arrangement. Chemists are familiar with the phenomena 

 exhibited when gases, solids, or liquids are exposed to each other under those circum- 

 stances where no direct change of composition ensues. Thus, if a cubic inch of carbonic 

 acid gas be exposed to a cubic inch of water, the gas in a short time passes into the 

 liquid mass, or is absorbed by it, with a certain degree of force and to a certain amount. 

 Also, aqueous gas rises from the water, and diffuses itself into the unabsorbed remainder 

 of the carbonic acid. After a sufficient time, no part of the carbonic acid will be found 

 destitute of aqueous gas, nor will any part of the water be without its equivalent of 

 carbonic acid. The simplest example of these combinations is furnished by the solution 

 of saline bodies in water, where there is no change of chemical composition, but merely 



