Prof. Dra2)er on the Chemical Action of Liyht. 369 



2. \Miat is the nature of the impression made on the material 

 group, the decomposition of which cnsiies ? 



These queries are of course connected with our theoretical 

 views of the nature of light, and of radiations generally. 



There are manj'^ facts, such as those to which attention has 

 been directed in the Philosophical Magazine (Febraary 1848), 

 which justify the supposition that the parts of all material sub- 

 stances are in a state of incessant vibration. To each particular 

 thermometric degree there belongs a particular frequency of vi- 

 bration. As soon as these motions approach four hundi'ed bil- 

 lions in a second, a red light is emitted, and the temperature is 

 near 1000° F. As the frequency increases, rays of a higher 

 refrangibility are in succession evolved, and the temperature cor- 

 respondingly rises. On the other hand, when these oscillatoiy 

 mo\ements decline, the temperature of the body falls. 



These principles lead to a ready explanation of the nature of 

 the exchanges of heat, and tlie cause of the equilibrium of tem- 

 perature. The ^ibratoly molecular motions are necessarily pro- 

 pagated to the ;ether, through which medium they are again 

 transferred to the particles of other bodies, on which the ethereal 

 waves impinge ; as a vibrating string excites undulations in the 

 ail-, and these in their turn can give birth to analogous motions 

 in other strings at a distance. 



There is an analogy between the relations of a hot and cold 

 body and those of two strings, one of which is emitting a musical 

 sound, and compelling the other to execute synchronous move- 

 ments. The jether in one case, and the air in the other, are the 

 media through which these motions pass. 



Equilibrium of temperature takes place when the molecules of 

 the substances concerned are in synclu-onous and equal vibration. 

 A hot body, in presence of a cold one, compels the latter to hasten 

 its rate of motion, its own rate all the time declining ; and this 

 continues luitil both have the same frequency ; then equilibrium 

 of temperature residts. The theory of the exchanges of heat is 

 therefore only an expression for the exchange of vibrations through 

 the jether. 



But temperature in thermotics is the equivalent term for bril- 

 liancy in optics. Both refer to compound qualities, depending 

 not only on frequency of \ibration, but also on its amplitude. As 

 the degree of heat of a mass rises, it expands ; the increase in its 

 volumcr indicating, that not only do tlie parts vibrate more swiftly, 

 but also that their individual excursions are increased. It fol- 

 lows, therefore, that every mass will have a determinate volume 

 for every degree of heat, a volume increasing as the temperature 

 rises. On this \ iew, the cx|)lanation of the expansion of bodies 

 by heat is, that these ])arts arc not only vibrating more quickly, 

 but also that the individual cxc\u'sions are greater. 



Pliil. M(i(f. S. 1. Vol. I. X.). ."). Minj IH.")!. :i. C 



