CAUSE OF SOLAR HEAT. 585 



words, there resides in mere matter, in what we have so long and so idly called 

 inert matter, the real source of every kind of movement, of every kind of life, 

 within the universe itself. For, what is true of our sun is true of his fellow suns, 

 the stars; not only of the thousands we see, but of the tens, the hundreds of 

 millions revealed by the telescope, and of the millions of millions of galaxies of 

 suns which doubtless exist beyond the domain surveyed by our most powerful 

 telescopes. 



But, turning from this stupendous, one may truly say, this awful mystery, 

 which seems to present gravitation as in a sense associated directly with the great 

 first cause, we note that there are many minor mysteries about the theory that 

 solar gravitation is the true source of solar heat. 



In the first place, many find it difficult to understand how the shrinkage of 

 even solid matter, still less that of vaporous matter, can lead to the generation of 

 heat. Yet in reality, to any one who rightly apprehends the principle of the con- 

 servation of energy, it will be obvious that the heat generated as the solar gravity 

 draws inward any portion of his envelope of vapors, and in so doing necessarily 

 diminishes the volume of that portion, must be exactly the equivalent of the heat 

 which would be required to reverse the process, and so to restore by expansion 

 that portion of vaporous matter to its original volume. A real difficulty arises for 

 a moment when we inquire why the heat generated momentarily by the forces 

 tending to produce contraction is not momentarily employed in producing 

 equivalent counter expansion. We must remember, however, that a portion of 

 the heat generated must necessarily be radiated away into space, simply because 

 it is exposed to the cold of space. One may compare the case to that of a pump 

 so constructed that if all the water raised remained in a certian- vessel round the 

 place of exit, the weight of water would serve as a source of power to keep the 

 pump working. In such a case, were friction entirely gotten rid of, the pump 

 would work forever. But if the vessel were perforated so that all the time a 

 portion of the water escaped, this would no longer happen. The heat constantly 

 generated by the enforced process of solar contraction does in part escape; a 

 portion continually undoes part of the work of contraction by causing expansion, 

 but the portion which, as we see and feel, is continually escaping from the sun, 

 causes a portion of the contraction to remain uncompensated. Thus the sun as 

 a whole, continues steadily contracting and steadily emitting heat. Nor will he 

 cease to contract until he ceases to emit light and heat, or in other words, until 

 he ceases to be the beneficent light-giving center of the solar system. 



But the most perplexing mystery in connection with the contraction theory 

 of the sun's heat is that the sun and the earth seem to tell a different story in 

 regard to the amount of heat which has been poured forth. Judging from the 

 sun's apparent size, it would seem as though not more than 20,000,000 of years 

 of solar work, at the sun's present rate of working, could possibly have been 

 done ; for if he had gathered in his mass from any distances, however large, that 

 would be the absolute maxim of energy represented by the process of contraction 

 to his present apparent size. But the earth's crust seems to tell us of a much 



