190 RADIATTOlSr IN THE SOLAR SYSTEM. 



torial teniperatiire can not be much higher than the average. On 

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

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

 establish far higher temperatures than the earth Avould have at the 

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

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

 canals. Unless he is very diflVrent from the earth, his whole surface 

 is below the freezing point. 



Let us now turn from these temperature efl'eets of radiation to an- 

 other class of effects, those due to pressure. 



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

 tromagnetic theory of light, light and all radiation like light 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 energy in a cubic centimeter of the stream. The exist- 

 ence of this pressure Avas fully demonstrated independently by Lebe- 

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

 in which they allowed a beaui 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 Avhich 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 

 press Avith a force of (5 by 10 ■' degree on a square centimeter, an 

 amount so small that on the AAliole earth it is but 75.000 tons, a mere 

 trifle compared Avith the -5,000,000 billion tons Avith Avhich the sun 

 pulls the earth by his gravitation. 



But noAV notice the remarkable effect of size on the relation betAA'een 

 the radiatiou pressur<> and the gravitative ])ull. One is on the sur- 

 face and proportional to the sm-fa(;e, while the otlxu- penetrates the 

 sni-face and pulls evei-y grain of nnitter throughout the whole volume. 



Siipi)ose Ave could divide the earth u]) into eight equal globes. Each 

 Avonid hav(^ half thedianieter of the (>arth and a (|uarter the sni'face. 

 The eight would expose twice the surface which the earth ex])oses, and 

 the total radiation ])r(>ssui-e Avonld be doubled, Avhile the total gravi- 

 tative pull Avould be the same as before. Noav divide up each of the 



