TBE COPLEY MED A List OP 1871. 331 
with its excitin-g cause, so the nerves, acting upon the 
muscles, can unlock an amount jof 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 1845, 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 
