and the Absolute Scale of Electric Potential. 21 



rapidly reversed electromotive forces were impressed upon it. 

 Now the impression of an electromotive force upon a film 

 possessing, as do all photochemically active bodies, electro- 

 lytic conductivity, will tend to produce at least partial elec- 

 trolysis. Hence the liberation of silver from its less stable 

 salts under the influence of light. The special power of the 

 waves of extreme refrangibility to produce these actions may 

 be due to the changes in vector-potential being relatively 

 more frequent for these than for the less-refrangible waves, 

 and consequently more rapid in proportion to the time of 

 vibration of the individual molecules. For if the period of 

 the vibrations of the rays be nearly in agreement with that 

 of the molecules of any kind of matter, the (electro-kinetic) 

 energy of the alternating electric displacements might pass 

 into the (pondero-kinetic) energy of calorific oscillations. 

 We know that the periods of the rays of lower refrangibility 

 agree more nearly with the vibration-periods of matter, and 

 that their absorption produces a rise of temperature. We 

 know also that the periods of the rays of higher refrangibility 

 are small in comparison, and that those rays do not in general 

 produce calorific oscillations. It may well be accepted that 

 instead they produce the more direct effect of impressing 

 alternate transient electromotive forces upon the quantities of 

 electricity with which the individual molecules appear to be 

 associated. 



9. Absolute Scale of Electrification. — If, then, electricity be 

 a single physical quantity, and subject to a law of conserva- 

 tion, it follows that one or other of the two known kinds of 

 electrification, either that called " vitreous " or "positive," 

 or else that called "resinous" or " negative," must consist 

 of a defect, or of an excess, above the ordinary distribution, or 

 average absolute charge, in the neighbouring bodies. Various 

 small specific differences have been observed from time to 

 time in the behaviour of the so-called positive and negative 

 distributions of electrification; but from no one of these is 

 it easy to draw any conclusions as to whether either the 

 " vitreous " or the " resinous " electrification really is the 

 excess. If there be two bodies at temperatures, one of which 

 may be above that of the surrounding atmosphere and the 

 other equally below, we can say, without hesitation, which of 

 them has an excess, and which a defect, of heat. If we could, 

 with equal certainty, say which of the two electric stares 

 really is due to an excess of electricity, and which to a defect, 

 we should at once be further led to expect the existence of 

 an absolute zero of electrification, which would bo the state of 

 a body deprived of all electricity. That none of the observed 



