xxiv.] EMPIEICAL KNOWLEDGE, EXPLANATION, &c. 549 



prove that they are exceedingly high. The phenomena 

 of elliptic polarisation, and perhaps also those of internal 

 radiation, 1 depend upon the refractive index, and thus, even 

 when we cannot observe any refracted rays, we can in- 

 directly learn how they would be refracted. 



In many cases large quantities of electricity must be 

 produced, which we cannot observe because it is instantly 

 discharged. In the common electric machine the cylinder 

 and rubber are made of non-conductors, so that we can 

 separate and accumulate the electricity. But a little damp, 

 by serving as a conductor, prevents this separation from 

 enduring any sensible time. Hence there is no doubt that 

 when we rub two good conductors against each other, for 

 instance two pieces of metals, much electricity is produced, 

 but instantaneously converted into some other form of 

 energy. Joule believes that all the heat of friction is 

 transmuted electricity. 



As regards phenomena of insensible amount, nature is 

 absolutely full of them. We must regard those changes 

 which we can observe as the comparatively rare aggregates 

 of minuter changes. On a little reflection we must allow 

 that no object known to us remains for two instants of 

 exactly the same temperature. If so, the dimensions of 

 objects must be in a perpetual state of variation. The 

 minor planetary and lunar perturbations are infinitely 

 numerous, but usually too small to be detected by observa- 

 tion, although their amounts may be assigned by theory. 

 There is every reason to believe that chemical and electric 

 actions of small amount are constantly in progress. The 

 hardest substances, if reduced to extremely small particles, 

 and diffused in pure water, manifest oscillatory movements 

 which must be due to chemical and electric changes, so 

 slight that they go on for years without affecting appreciably 

 the weight of the particles. 2 The earth's magnetism must 

 more or less affect every object which we handle. As 

 Tyndall remarks, " An upright iron stone influenced by the 

 earth's magnetism becomes a magnet, with its bottom a 

 north and its top a south pole. Doubtless, though in an 

 immensely feebler degree, every erect marble statue is a 



1 Balfour Stewart, Elementary Treatise en Heat, ist edit. p. 198. 



2 Jevons, Proceedings of the Manchester Literary and Philosophical 

 Society, 25th January, 1870, vol. ix. p. 78. 



