242 Dr. J. It. Mayer on Celestial Dynamics. 



this body kept up in undiminished force so as to enable him to 

 send forth his rays into the universe in such a grand and mag- 

 nificent manner ? What are the causes which counteract or 

 prevent his exhaustion, and thus save the planetary system from 

 darkness and deadly cold ? 



Some endeavoured to approach " the grand secret/' as Sir Wm. 

 Herschel calls this question, by the assumption that the rays of 

 the sun, being themselves perfectly cold, merely cause the " sub- 

 stance " of heat, supposed to be contained in bodies, to pass from 

 a state of rest into a state of motion, and that in order to send 

 forth such cold rays the sun need not be a hot body, so that, in 

 spite of the infinite development of light, the cooling of the sun 

 was a matter not to be thought of. 



It is plain that nothing is gained by such an explanation ; for, 

 not to speak of the hypothetical " substance " of heat, assumed 

 to be at one time at rest and at another time in motion, now 

 cold and then hot, it is a well-founded fact that the sun does 

 not radiate a cold phosphorescent light, but a light capable of 

 warming bodies intensely ; and to ascribe such rays to a cold 

 body is at once at variance with reason and experience. 



Of course such and similar hypotheses could not satisfy the 

 demands of exact science, and I will therefore try to explain in 

 a more satisfactory manner than has been done up to this time 

 the connexion between the sun's radiation and its effects. In 

 doing so, I have to claim the indulgence of scientific men, who 

 are acquainted with the difficulties of my task. 



II. Sources of Heat. 



Before we turn our attention to the special subject of this 

 paper, it will be necessary to consider the means by which light 

 and heat are produced. Heat may be obtained from very dif- 

 ferent sources. Combustion, fermentation, putrefaction, slaking 

 of lime, the decomposition of chloride of nitrogen and of gun- 

 cotton, &c. &c, are all of them sources of heat. The electric 

 spark, the voltaic current, friction, percussion, and the vital 

 processes are also accompanied by the evolution of this agent. 



A general law of nature, which knows of no exception, is the 

 following: — In order to obtain heat, something must be expended; 

 this something, however different it may be in other respects, 

 can always be referred to one of two categories : either it consists 

 of some material expended in a chemical process, or of some sort 

 of mechanical work. 



When substances endowed with considerable chemical affinity 

 for each other combine chemically, much heat is developed 

 during the process. We shall estimate the quantity of heat 

 thus set free by the number of kilogrammes of water which it 



