188 THE PRINCIPLES OF SCIENCE. 



observation, although their amounts may be confidently 

 assigned by theory. There is every reason to believe 

 that chemical and electric actions of almost indefinitely 

 small amount, are constantly in progress. The hardest 

 and most fixed substances, if reduced to sufficiently small 

 particles, and diffused in pure water, manifest oscillatory 

 movements which must be due to chemical and electric 

 changes, so slight that they may go on for years without 

 affecting appreciably the weight of the particles. The 

 earth's magnetism must affect more or less every object 

 which we handle. As Professor 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 true diamagnet, 

 with its head a north pole and its feet a south pole. The 

 same is certainly true of man as he stands upon the 

 earth's surface, for all the tissues of the human body are 

 diamagnetic 1 .' The sun's light produces a very quick 

 and perceptible effect upon the photographic plate ; in all 

 probability it has a much less effect upon a great variety 

 of substances. We may regard every apparent pheno- 

 menon as but an exaggerated and conspicuous case of a 

 process which is, in indefinitely more numerous cases, 

 beyond the means of observation. Yet in a great pro- 

 portion of these cases exact calculation will enable us to 

 estimate the amount of the phenomena, if it is of suf- 

 ficient interest for us to do so. 



i < 



Philosophical Transactions,' vol. cxlvi. p. 249. 



