NATURAL PHILOSOPHY. 105 



while to pay attention to this point, and to ascertain whether rifle 

 bullets do not, under some circumstances, show signs of fusion. 



The conversion of muscular force into heat is strikingly shown in 

 the concussion of flint and steel, as in the old method of obtaining a 

 light. Energetic chemical union is always attended with the evolu- 

 tion of heat, which may be regarded as being produced by the falling 

 together of atoms at a high velocity. The heat so evolved can be 

 made to reproduce the exact amount of force that was arrested in its 

 production. The burning of charcoal in oxygen is an old and famil- 

 iar experiment; but it now has a significance beyond what it for- 

 merly had, inasmuch as we may now regard the act of combination 

 on the part of the atoms of oxygen and coal exactly as we regard the 

 clashing of a falling weight against the earth. And the heat pro- 

 duced in both cases is referable to a common cause. This glowing 

 diamond, which burns in oxygen as a star of white light, glows and 

 burns in consequence of the falling of the atoms of oxygen against 

 it. And could we measure the velocity of the atoms when they 

 clash, and could we find their number and weight, multiplying the 

 mass of each atom by the square of its velocity, and adding all 

 together, we should get a number representing the exact amount of 

 heat developed by the union of the oxygen and carbon. 



Thus far we have regarded the heat developed by the clashing of 

 sensible masses and of atoms. Work is expended in giving motion 

 to these atoms or masses, and heat is developed. But we reverse 

 this process daily, and by the expenditure of heat execute work. 

 We can raise a weight by heat, and in this agent we possess an enor- 

 mous store of mechanical power. Thus, the union of a pound of 

 coal with about two pounds of oxygen evolves an amount of heat 

 capable, if properly applied, of raising a hundred pounds' weight to 

 a height of twenty miles. The coal raised every year in England 

 amounts to 84,000,000 tons, which, were they all applied to the pro- 

 duction of force, would be equal to 108,000,000 horses working 1 con- 

 stantly : or a pound of coal may be regarded as equal to the force of 

 three hundred horses working for one minute. Conversely, one hun- 

 dred pounds falling from a height of twenty miles, and striking 

 against the earth, would generate an amount of heat equal to that 

 developed by the combustion of a pound of coal. Wherever work is 

 done by heat, heat disappears. A gun which fires a ball is less heated 

 than one which fires blank cartridge. The quantity of heat com- 

 municated to the boiler of a working steam engine is greater than 

 that which could be obtained from the recondensation of the steam 

 after it had done its work ; and the amount of work performed is the 

 exact equivalent of the amount of heat lost. 



The fact that force may be converted into heat has given rise to a 

 theory which attributes the light and heat evolved from the sun to 

 the falling of meteoric bodies on to its surface. Of the amount of 

 heat produced by this luminary some idea may be gained from the 

 fact that the earth receives only -23-0-0 ^ooooo" P art - Moreover, it is 

 calculated that the heat given out by the sun every minute is sufficient 

 to boil 12,000,000 cubic miles of ice-cold water. This vast amount 

 of heat is supposed by Dr. Mayer, of Heilbron, to be due to the fall- 

 ing into the sun of meteorites. The objection to the theory that 



