109 



of a second. The periods of the vibrations of visible light lie between 

 this limit and another, about double as great, corresponding to the ex- 

 treme visible red light. The vibrations of the obscure radiant heat 

 beyond the red end are executed in longer periods than this ; the 

 longest which has yet been experimentally tested being about the 

 eighty million millionth of a second. 



The elevation of temperature produced in a body by the incidence of 

 radiant heat upon it is a mechanical effect of the dynamical kind, since 

 the communication of heat to a body is merely the excitation or the 

 augmentation of certain motions among its particles. According to 

 Pouillet's estimate of heat radiated from the sun in any time, and 

 Joule's mechanical equivalent of a thermal unit, it appears that the 

 mechanical value of the solar heat incident perpendicularly on a square 

 foot above the earth's atmosphere is about eighty-four foot-pounds 

 per second. 



Mechanical effect of the statical kind might be produced from the 

 solar radiant heat, by using it as the source of heat in a thermo- 

 dynamic engine. It is estimated that about 556 foot-pounds per 

 second of ordinary mechanical effect, or about the work of " one horse 

 power,'' might possibly be produced by such an engine exposing 1800 

 square feet to receive solar heat, during a warm summer day in this 

 country ; but the dimensions of the moveable parts of the engine 

 would necessarily be so great as to occasion practical difficulties in the 

 way of using it with economical advantage that might be insurmount- 

 able. 



The chemical effects of light belong to the class of mechanical 

 effects of the statical kind ; and reasoning analogous to that intro- 

 duced and experimentally verified in the case of electrolysis by Joule, 

 leads to the conclusion that when such effects are produced there will 

 be a loss of heating effect in the radiant heat or light which is 

 absorbed by the body acted on, to an extent thermally equivalent to 

 the mechanical value of the work done against forces of chemical 

 affinity. 



The deoxidation of carbon and hydrogen from carbonic acid and 

 water, effected by the action of solar light on the green parts of plants, 

 is (as the author recently found was pointed out by Helmholz* in 

 1847), a mechanical effect of radiant heat. In virtue of this action 



* " Ueber die Erhaltung der Kraft, von Dr H. Hclmhol«." Berlin, 1847. 



