THE EARTH'S ABSORPTION OF HEAT. 75 



effects of these rays by the ordinary state of the medium 

 through which the solar heat passes. Our atmosphere 

 is a mixture of gases and aqueous vapour ; and it has 

 been found, as already stated, that even a thin film of 

 water, however transparent, prevents the passage of many 

 calorific radiations, and the rays ictarded are, for the 

 most part, of that class which have this peculiar scorch- 

 ing power. The air is, in this way, the great equaliser 

 of the solar heat, rendering the earth agreeable to all 

 animals, who, but for this peculiar absorbent medium, 

 would have to endure, even in our temperate clime, the 

 burning rays of a more than African sun. 



The surface of the earth during the sunshine and, 

 though in a less degree, even when the sun is obscured 

 by clouds is constantly receiving heat ; but the rate of 

 its absorption varies. Benjamin Franklin showed, by a 

 set of simple but most conclusive experiments, that a 

 piece of black cloth was warmed much sooner than cloth 

 of a lighter colour;* and we know, from observations 

 of a similar class, that the bare brown soil receives heat 

 more readily than the bright green grassy carpet of the 

 earth. Consequently, during the winter season, rela- 



scribed in a former communication (Phil. Trans, vol. cxxxiii.); and 

 On the Parathermic Rays of the Solar Spectrum, Phil. Trans, vol. 

 cxxxiv. 



The experiments of Mrs. Somerville, On the Action of the Rays 

 of the Spectrum on Vegetable Juices (Phil. Transactions, vol. 

 cxxxvii.), appear to connect themselves with this particular class 

 of rays in a curious manner. 



* Experiments on the influence of heat on differently-coloured 

 bodies were first made by Dr. Hooke ; and it was not until long 

 after that Franklin made his ingenious experiments. Davy exposed 

 to sunshine six equal pieces of copper, painted white, yellow, red, 

 green, blue, and black, in such a manner that one side only was 

 illuminated, To the dark side he attached a bit of cerate, ascer- 

 tained by experiment to melt at 700. The cerate attached to the 

 black became fluid first, the blue next, then the green and red, and 

 lastly the yellow and white. Beddoes's Contributions to Physical 

 Knowledge, and collected works of Sir Humphry Davy, vol. ii. p. 27. 



