228 



account for the phenomena of polarisation. But all observed 

 phenomena of light are in perfect harmon}^ with the undulatory 

 theory, as now commonly accepted; and not only does this 

 theory fit all previous observations, but the appearance that 

 will be presented to the eye, when a ray of light is transmitted 

 through any hitherto untried arrangement of transparent 

 media, may safely be predicted by analysis, as in the remark- 

 able case of Airy^s spirals, se6n when a polarised ray is 

 successively transmitted through two plates cut from a right 

 and a left-handed quartz crystal. Caloric was once assumed to 

 be the matter or substance of heat ; and the observed radiation 

 of cold induced Black to ascribe to cold an independent material 

 existence; but the observed phenomena are completely explic- 

 able on the "theory of exchanges/^ which means that every 

 body radiates its own temperature, whether high or low, and 

 that every surrounding body absorbs the radiations ; conse- 

 quently, the radiations of a cold body will lower the heat of a 

 warmer body in its vicinity, just as a cistern with two pipes of 

 unequal bore will, if fed by the larger and emptied by the 

 smaller, become gradually fuller, while if fed by the smaller 

 and emptied by the larger_, its contents will be diminished : the 

 parallel, in absorbed and emitted radiations, is obvious. 



31. Again, it was formerly taught that there were two electric 

 "fluids" — the "vitreous" and "resinous"; but these were 

 subsequently merged into one, and the positive and negative 

 aspects of electricity were assumed to be differences in quantity 

 only, and not in kind, positively electrified bodies being in 

 excess, and those negatively electrified, in defect of the normal 

 quantity. Moreover, magnetic properties were supposed to be 

 vested in two "fluids" — the ^'austral" and "boreal" — pos- 

 sessing mutually attractive and self-repulsive properties. But 

 all these theories are more than probably alike groundless ; they 

 are, moreover, utterly inconsistent Avith the perpetually re- 

 curring interchanges of the various presumed forms of energy ; 

 for it is impossible to conceive one Idnd of matter to be con- 

 verted into another kind, or matter to be converted into mere 

 motion, and vice versa. 



32. In the vibratory motions of the atmosphere and other 

 bodies, which convey to the ear the impression of sound, the vibra- 

 tions are demonstrably longitudinal; that is, the vibratory motion 

 of each particle is in the direction in which the wave is travel- 

 ling, as in the wave the wind produces in a field of corn : in the 

 vibrations of light and heat, the phenomena of polarisation 

 require that they must be transverse, that is, the vibration of 

 each particle must be in a plane perpendicular to the direction 

 of the wave, as in the ripples on the surface of still water. 



