66 



DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 



that they are dependent upon the natural conditions of gaseous elasticity combined with 

 the exertion of that attractive force, possessed by many bodies in an eminent degree, 

 and probably belonging to all, by which they are drawn into association more or less 

 close, without at the same time undergoing chemical combination, though often 

 assuming the condition of adhesion ; and which occasionally leads, under very favour- 

 able circumstances, as in the present instance, to the combination of bodies simultane- 

 ously subjected to this attraction. I am prepared myself to admit (and probably many 

 others are of the same opinion), both with respect to the attraction of aggregation and 

 of chemical affinity, that the sphere of action of particles extends beyond those other 

 particles with which they are immediately and evidently in union, and in many cases 

 produces effects rising into considerable importance : and I think that this kind of 

 attraction is a determining cause of Dobereiner's effect, and of the many others of 

 a similar nature. 



620. Bodies which become wetted by fluids with which they do not combine che- 

 mically, or in which they do not dissolve, are simple and well known instances of this 

 kind of attraction. 



62 1 . All those cases of bodies which being insoluble in water and not combining 

 with it are hygrometric, and condense its vapour around or upon their surface, are 

 stronger instances of the same power, and approach a little nearer to the cases under 

 investigation. If pulverised clay, protoxide or peroxide of iron, oxide of manganese, 

 charcoal, or even metals, as spongy platina or precipitated silver, be put into an at- 

 mosphere containing vapour of water, they soon become moist by virtue of an attrac- 

 tion which is able to condense the vapour upon, although not to combine it with, the 

 substances ; and if, as is well known, these bodies so damped be put into a dry 

 atmosphere, as, for instance, one confined over sulphuric acid, or if they be heated, 

 then they yield up this water again almost entirely, it not being in direct or per- 

 manent combination*. 



622. Still better instances of the power I refer to, because they are more analogous 

 to the cases to be explained, are furnished by the attraction existing between glass 

 and air, so well known to barometer and thermometer makers, for here the adliesion 

 or attraction is exerted between a solid and gases, bodies having very different phy- 

 sical conditions, having no power of combination with each other, and each retaining, 

 during the time of action, its physical state unchanged -f-. When mercury is poured 

 into a barometer tube, a film of air will remain between the metal and glass for 

 months, or, as far as is known, for years, for it has never been displaced except by the 



* I met at Edinburgh with a remarkable case as to its extent of hygrometric action, assisted a little perhaps 

 by very slight solvent power. Some turf had been well dried by long exposure in a covered place to the atmo- 

 sphere, but being then submitted to the action of a hydrostatic press, it yielded, by the mere influence of the 

 pressure, 54 per cent, of water. 



t FusiNiERi and Bellani consider the air as forming solid concrete films in these cases. — Griornale di Fisica, 

 torn. viii. p. 262. 1825. 



