34 CHEMICAL ACTION OF LIGHT. PAST z. 



The amount of the chemical action of light has been 

 determined by Professor Eoseoe to be directly propor- 

 tional to the intensity of the light ; and when the light 

 is constant the amount of action is exactly proportional 

 to the time of exposure. It appears that equal volumes 

 of chlorine and hydrogen explode in sunshine, but 

 combine slowly in shade ; and as the combined gases 

 are absorbed by water as soon as combined, the gradual 

 diminution of the volume of the mixed gases during 

 the time of absorption is a measure of the amount of 

 action exerted by the light. 



Professor Wm. Thomson has computed, by the aid of 

 Poullet's data of solar radiations and Mr, Joule's me- 

 chanical equivalent of heat, that the mechanical value 

 of the whole energy, active and potential, of the dis- 

 turbances kept up on the ethereal medium by the 

 vibrations of the solar light in a cubic mile of our 

 atmosphere, is equal to 12,050 times the unit of me- 

 chanical force : that is to say, twelve thousand and 

 fifty times the force that would raise a pound weight of 

 matter to the height of one foot. The sensible height 

 of the atmosphere is about forty miles, whence some 

 idea may be formed of the vast amount of force exerted 

 by the sun's light within the limits of the terrestrial 

 atmosphere. The green mantle which clothes the earth 

 proves under a beautiful form the influence of light on 

 the organic world. 



It has been proved that at any given fixed temperature 

 the amount of light and heat absorbed and that which is 

 emitted remains constant for all bodies. The greater the 

 amount absorbed, the greater the amount radiated. The 

 molecules or atoms of the bodies in consequence of the 

 law of resonance emit those ethereal undulatory motions 

 which have been previously impressed upon them, as a 



