50 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I. 



have just observed in the progression of quality. These 

 researches then conduct, as do others heretofore known 

 on light and radiant heat, to a perfect analogy between 

 the general laws which govern these two great agents of 

 nature." 



The law of the radiation of heat, as illustrated by the 

 foregoing experiments with an ignited strip of platinum, 

 has been applied in recent discussions respecting the age 

 of the earth. Geological evidence has satisfactorily es- 

 tablished that the temperature of the earth was formerly 

 much higher than now, and the decline that has hap- 

 pened could only have taken place by radiation into 

 space. Considering how slow the cooling now is a 

 scarcely perceptible fraction of a degree in the course of 

 many centuries it would seem that to accomplish the 

 whole descent, if even w r e go no further back than the 

 paleozoic era, an amazing lapse of time would be re- 

 quired. And if we accept the nebular hypothesis, since 

 the original temperature must have been at least that of 

 the surface of the sun, the time must be correspondingly 

 extended. Even if numbers could be given, the ima^ina- 



o f O 



tion would altogether fail to appreciate them. 



But we have here experimental proof that the higher 

 the temperature of a body, the more rapidly it cools. A 

 descent through a given number of degrees is more quick- 

 ly made when a body is at a high than when it is at a 

 low temperature. Anciently the cooling of the earth 

 was more rapid than it is now. Not that there was any 

 change or breach in the general law under which the op- 

 eration was taking place, for the same mathematical ex- 

 pression applies to all temperatures, no matter how high 

 or how low they may be. Mr. Croll, in his recent re- 

 searches on the distribution of heat over the globe, points 

 out the bearing of these experiments. 



