THE SUN'S HEAT. ii 



established, we may anticipate that more special ones can be alleged 

 only as probable. 



Happily, however, as we shall eventually see, those general con- 

 clusions admitting of positive establishment are the conclusions of 

 most value for guidance. 



THE SUN'S HEAT. 



By Professor C. A. YOUNG, 



OF PRINCETON, N. J. 



THERE has been a prevailing idea for many years, founded upon 

 Brewster's fallacious experiments, that thermal, luminous, and 

 chemical rays are fundamentally different, though coexistent in the 

 sunbeams. This is erroneous : it is true, indeed, that rays whose vi- 

 brations are too slow to be seen produce powerful heating effects, and 

 that those which are invisible because they are too raj^id have a strong 

 influence in determining certain chemical and physical reactions ; but 

 it is also true that the visible rays are capable of producing the same 

 effects to a greater or less degree, and there is some reason for think- 

 ing that certain animals can see by rays to which the human retma is 

 insensible. There is absolutely no philosophical basis for distinction 

 between the visible and invisible radiations of the sun, except in the 

 one point of vibration-frequency their ^j>z*^c7i, to use the analogy of 

 sound. The expressions thermal, luminous, and chemical rays are 

 apt to be misleading. All the waves of solar radiation are carriers of 

 energy, and when intercepted do work, producing heat, or vision, or 

 chemical action, according to circumstances. 



If the amount of solar light is enormous as compared with terres- 

 trial standards, the same thing is still more true of the solar heat, 

 which admits of somewhat more accurate measurement, since we are 

 no longer dependent on a so imsatisfactory unit as the "candle- 

 power," and can substitute thermometers and balances for the human 

 eye. 



It is possible to intercept a beam of sunshine of known dimensions, 

 and make it give up its radiant energy to a weighed mass of water or 

 other substance, to measure accurately the rise of temperature pro- 

 duced in a given time, and from these data to calculate the whole 

 amount of heat given off by the sun in a minute or a day. 



Pouillet and Sir John Herschel seem to have been the first fairly 

 to grasp the nature of the problem, and to investigate upon the sub- 

 ject in a rational manner. 



Herschel's experiments were made in 1838 at the Cape of Good 

 Hope, where he was then engaged in his astronomical work. He pro- 

 ceeded in this way : A small tin vessel, containing about half a pint 



