34 IN STARRY REALMS. 



sufficient to supply its entire radiation for two thousand 

 years. 



If it be the case that the sun's temperature does remain 

 absolutely constant, then it would seem to follow that the 

 diminution of one ten-thousandth part of its diameter 

 takes place every two thousand years. "We have no means 

 of knowing at present whether the actual contraction of 

 the sun takes place at this rate or at any other rate, either 

 somewhat higher or somewhat lower. If the contraction 

 goes on faster than we have stated, then the temperature 

 of the sun must be rising ; if slower, then the temperature 

 must be falling. 



Here, then, we have a completely satisfactory explana- 

 tion as to how the sun is able to continue from age to age 

 its beneficent radiation. So long as the body is sufficiently 

 gaseous, so long will it obey the laws of cooling we have 

 indicated, and so long may the amount of its radiation be 

 undiminished. We have seen, however, that there must 

 be some limit to this process. As the contraction of 

 the sun's volume proceeds, the density of the body will 

 increase to such a degree that the luminary is less and 

 less to be regarded as a gaseous mass. At length the 

 time will come when the sun shall have parted with so 

 much heat that it passes to a large extent into the solid 

 condition. There can be no doubt that when this 

 state of things has arrived the heat-dispensing power of 

 the sun will be seriously impaired. The radiation from 

 a solid, as we have already had occasion to show, could 

 not be protracted with sensible uniformity from age to 

 age. We thus have no assurance of the ultimate perma- 

 nence Tif the sun as a source of heat to our system. Indeed 



