190 RADIATION IN THE SOLAR SYSTEM. 



torial teinper.iliii'e can not be much higher than the average. On 

 certain suppositions I find that it is still 20° beloAv the freezing point, 

 and until some new conditions can be pointed out which enable him to 

 establish far higher temperatures than the earth would have at the 

 same distance it is hard to belicAe that he can have polar caps of 

 frozen water melting to li(|uid in his summer and filling rivers or 

 canals. Unless he is very ditl'erent from the earth, his whole surface 

 is below the freezing point. 



Let us noAv turn from these temperature effects of radiation to an- 

 other class of effects, those due to pressure. 



More than thirty years ago Clerk Maxwell showed that on his elec- 

 tromagnetic theory of light, light and all radiation like liglit should 

 press against any surface on which it falls. There should also be a 

 pressure back against any surface from which radiation is reflected 

 or from which it is issuing as a source, the value in every case being 

 equal to the energ^^ in a cubic centimeter of the stream. The exist- 

 ence of this pressure was fully demonstrated independently by Lebe- 

 dew and by Nichols and Hull some years ago in brilliant experiments 

 in which they allowed a beam of light to fall on a suspended disk in 

 a vacuum. The disk was repelled, and they measured the repulsion 

 and found it to be about that required by Maxwell's theory. Nichols 

 and Hull have since repeated the experiment with greater exactness, 

 and there is now no doubt that the pressure exists and that it has 

 Maxwell's value. 



The radiation, then, poured out by the sun is not only a stream of 

 energy, but it is also, as it were, a stream of pressure pressing out the 

 heavenly bodies on Avhicli it falls. Since the stream thins out as it 

 diverges, according to the inverse square of the distance, the pressure 

 on a given surface falls off according to the same law. We know the 

 energy in a cubic centimeter of sunlight at the distance of the earth, 

 since, moving with the velocity of light, it will supply one twenty- 

 fourth of a calory per second. It is easy to calculate that it will 

 j)ress with a force of 6 b}^ 10 "' degree on a square centimeter, an 

 amount so small that on the whole earth it is but 75,000 tons, a mere 

 trifle compared with the 8,000,000 billion tons with which the sun 

 pulls the earth by his gravitation. 



But now notice the remarkable effect of size on the relation between 

 the radiation pressure and the gravitative i)ull. One is on the sur- 

 face and |)ro])ortional to the surface, while the other penetrates the 

 surface and pulls every grain of matter throughout the whole volume. 



Supi)ose we could divide the eai'th up into eight equal glolx's. Each 

 would ha\'e half the diameter of the earth and a (piarter the surface. 

 The eight would expose twice the surface which the earth exposes, and 

 the total radiation pressure would be doubled, while the total gravi- 

 tative pull would be the same as before. Now divide up each of the 



