COPLEY MKDALIM OF mi. 331 



with its exciting cause, so the nerves, acting upon the 

 muscles, can unlock an amount of activity, wholly out of 

 proportion to the work done by the. nerves themselves. 



As regards these questions of weightiest import to the 

 science of physiology, Dr. Mayer, in 18-45, was assuredly 

 far in advance of all living men. 



Mayer grasped the mechanical theory of heat with com- 

 manding 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 the solar heat. He followed the energies 

 which were potential in the vegetable, 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 degrees 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 difficulties attend- 

 ing 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 degrees 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 the truth of this theory; 

 that is a question which may still have to be fought out. 

 But I refer to it as an illustration of the force of genius 

 with which Mayer followed the mechanical theory of heat 



