THEORY OF THE COMBINATION OF GASES BY PLATINA. 67 



action of means especially fitted for the purpose. These consist in boiling the mercury, 

 or, in other words, of forming an abundance of vapour, which coming in contact with 

 every part of the glass and every portion of surface of the mercury, gradually mingles 

 with, dilutes, and carries off the air attracted by, and adhering to, those surfaces, re- 

 placing it by other vapour, subject to an equal or perhaps greater attraction, but which 

 when cooled condenses into the same liquid as that with which the tube is filled. 



623. Extraneous bodies, which, acting as nuclei in crystallizing or depositing 

 solutions, cause deposition of substances on them, when it does not occur elsewhere 

 in the liquid, seem to produce their effects by a power of the same kind, i. e. a power 

 of attraction extending to neighbouring particles, and causing them to become 

 attached to the nuclei, although it is not strong enough to make them combine 

 chemically with their substance. 



624. It would appear from many cases of nuclei in solutions, and from the effects 

 of bodies put into atmospheres containing the vapours of water, or camphor, or 

 iodine, &c., as if this attraction were in part elective, partaking in its characters 

 both of the attraction of aggregation and chemical affinity : nor is this inconsistent 

 with, but agreeable to, the idea entertained, that it is the power of particles acting, 

 not upon others with which they can immediately and intimately combine, but upon 

 such as are either more distantly situated with respect to them, or which, from 

 previous condition, physical constitution, or feeble relation, are unable to enter into 

 decided union with them. 



625. Then, of all bodies, the gases are those which might be expected to show some 

 mutual action whilst jointly under the attractive influence of the platina or other 

 solid acting substance. Liquids, such as water, alcohol, &c., are in so dense and 

 comparatively incompressible a state, as to favour no expectation that their particles 

 should approach much closer to each other by the attraction of the body to which 

 they adhere, and yet that attraction must (according to its effects) place their parti- 

 cles as near to those of the solid wetted body as they are to each other, and in many 

 cases it is evident that the former attraction is the stronger. But gases and vapoui-s 

 are bodies competent to suffer very great changes in the relative distances of their 

 particles by external agencies ; and where they are in immediate contact with the 

 platina, the approximation of the particles to those of the metal may be very great. 

 In the case of the hygrometric bodies referred to (621.), it is sufficient to reduce 

 the vapour to the fluid state, frequently from atmospheres so rare that without this 

 influence it would be needful to compress them by mechanical force into a bulk not 

 more than -jVth or even -gVth of their original volume before the vapours would be- 

 come liquids. 



626. Another most important consideration in relation to this action of bodies, and 

 which, as far as I am aware, has not hitherto been noticed, is the condition of elasti- 

 city under which the gases are placed against the acting surface. We have but very 

 imperfect notions of the real and intimate conditions of the particles of a body exist- 



k2 



