THE COPLEY MEDALIST OF 1871. 489 



Mayer grasped the mechanical theory of heat with 

 commanding power, illustrating it and applying it in 

 the most diverse domains. He began, as we have seen, 

 with physical principles ; he determined the numerical 

 relation between heat and work; he revealed the source 

 of the energies of the vegetable world, and showed the 

 relationship of the heat of our fires to solar heat. He 

 followed the energies which were potential in the vege- 

 table, up to their local exhaustion in the animal. But 

 in 1845 a new thought was forced upon him by his 

 calculations. He then, for the first time, drew attention 

 to the astounding amount of heat generated by gravity 

 where the force has sufficient distance to act through. 

 He proved, as I have before stated, the heat of collision 

 of a body falling from an infinite distance to the earth, 

 to be sufficient to raise the temperature of a quantity 

 of water, equal to the falling body in weight, 17,356 C. 

 He also found, in 1845, that the gravitating force 

 between the earth and sun was competent to generate 

 an amount of heat equal to that obtainable from the 

 combustion of 6,000 times the weight of the earth of 

 solid coal. With the quickness of genius he saw that 

 we had here a power sufficient to produce the enormous 

 temperature of the sun, and also to account for the 

 primal molten condition of our own planet. Mayer 

 shows the utter inadequacy of chemical forces, as we 

 know them, to produce or maintain the solar temperature. 

 He shows that were the sun a lump of coal it would be 

 utterly consumed in 5,000 years. He shows the diffi- 

 culties attending the assumption that the sun is a cooling 

 body; for, supposing it to possess even the high specific 

 heat of water, its temperature would fall 15,000 in 

 5,000 years. He finally concludes that the light and 

 heat of the sun are maintained by the constant impact 

 of meteoric matter. I never ventured an opinion as to 



