of Radiant Heat or Light. 257 



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 (Econo- 

 mical advantage that might be insurmountable. 



The chemical effects of light belong to the class of mechanical 

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

 troduced and experimentally vei'ified 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 combustible substances are produced by 

 plants ; and its mechanical value is to be estimated by deter- 

 mining the heat evolved by burning them, and multiplying by 

 the mechanical equivalent of the thermal unit. Taking, from 

 Liebig's Agricultural Chemistiy, the estimate 2600 pounds 

 of dry fir-wood for the annual produce of one Hessian acre, or 

 26,910 square feet of forest land (which in mechanical value 

 appears not to differ much from estimates given in the same 

 treatise for produce of various kinds obtained from cultivated 

 land), and assuming, as a very rough estimate, 4000 thermal 

 units Centigrade as the heat of combustion of unity of mass of 

 dry fir- wood, the author finds 550,000 foot-pounds (or the 

 work of a horse-power, for 1000 seconds) as the mechanical value 

 of the mean annual produce of a square foot of the land. Taking 

 50° 34' (that of Giessen) as the latitude of the locality, the author 

 estimates the mechanical value of the solar heat which, were none 

 of it absorbed by the atmosphere, would fall annually on each 

 square foot of the land, at 5.'50,000,000 foot-pounds ; and infers 

 that probably a good deal more, -j-(j'^(j of the solar heat, which 

 actually falls on growing plants, is converted into mechanical 

 effect. 



* Ueber die Erhaltung cler Kraft, von Dr. II. Helmholz. Berlin, 1847. 

 [A translation of this essay will iijipear in the First Part of the New 

 iSeries of the Scientific Memoirs. — Ed.] 



P/iil. May. S. 4. Vol. 4. No. 25. Oct. 1852. S 



