ON FORCE 393 



being the result. Here the atoms virtually fell together, 

 the amount of heat produced showing the enormous force 

 of the collision. The burning of charcoal in oxygen is 

 an old experiment, but it has now a significance beyond 

 what it used to have; we now regard the act of combina- 

 tion on the part of the atoms of oxygen and coal as we 

 regard the clashing of a falling weight against the earth. 

 The heat produced in both cases is referrible to a common 

 cause. A 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 meas- 

 ure the velocity of the atoms when they clash, and could 

 we find their number and weights, multiplying the weight 

 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 ex- 

 pended 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. A pound of coal produces 

 by its combination with oxygen an amount of heat which, 

 if mechanically applied, would suffice to raise a weight 

 of 100 Ibs. to a height of 20 miles above the earth's sur- 

 face. Conversely, 100 Ibs. falling from a height of 20 

 miles, and striking against the earth, would generate an 

 amount of heat equal to that' developed by the combus- 

 tion of a pound of coal. Wherever work is done by heat, 

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



