HOW THE HEAT IS KEPT UP. 31 



ing space. The loss of heat will therefore be at first a 

 very slow process, each loss involves a corresponding con- 

 traction of the volume, and as we have already pointed 

 out, this contraction must be attended with a rise of tem- 

 perature. As the temperature of the mass increases the 

 rate at which it parts with its heat will increase also ; it 

 follows that the contraction of the volume will proceed at 

 an accelerated pace, and that consequently the rising of 

 temperature will go on with increasing rapidity. We 

 thus find that though the temperature of the gas may 

 have been at first extremely low, yet, that as the loss of 

 heat proceeds the temperature may gradually ascend, 

 until at last it becomes sufficiently high to render the gas 

 visible by actual incandescence. As the process advances 

 still further the body may pass from a mere nebula into 

 a starlike object. With the increase of contraction the 

 pressure also increases, and materials which were origin- 

 ally gaseous will assume more and more a density resem- 

 bling that of solid bodies. 



Here is indeed an astonishing result. We have found 

 that a nebula which may have been originally only a 

 single degree of temperature above the surrounding 

 medium has by the mere fact that it is losing heat grown 

 to the lustre of a star, and may have acquired the heat- 

 diffusing power of a sun. Of course, the capacity for 

 radiation depends on temperature rather than on amount 

 of heat. 



The limit to this remarkable evolution has, however 

 to be stated. When the condensation has progressed 

 sufficiently for the body to have become transformed from 

 the gaseous state into a condition resembling that of a 



