530 Professor Tyndall [June 6, 



the speaker passed to the motion of atoms towards each other by chemical 

 affinity. A collodion balloon filled with a mixture of chlorine and 

 hydrogen was hung in the focus of a parabolic mirror, and in the focus 

 of a second mirror 20 ft. distant a strong electric light was suddenly 

 generated ; the instant tlie light fell upon the balloon, the atoms within 

 it fell together with explosion, and hydro-chloric acid was the result. 

 The burning of charcoal in oxygen was an old experiment, but it had 

 now a significance beyond what it used to have ; we 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 produced in both cases is referable to a common cause. This glow- 

 ing 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 Tiy the expenditure of heat execute work. We can 

 raise a weight by heat ; and in this agent we possess an enormous store 

 of mechanical power. This pound of coal, which I hold in my hand, 

 produces by its combination with oxygen an amount of heat which, 

 if mechanically applied, would suffice to raise a weight of 100 lbs. to 

 a height of 20 miles above the earth's surface. Conversely, 100 lbs. 

 falling from a height of 20 miles, and striking against the earth, would 

 genera^;e 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 communicated to the boiler of a 

 working steam-engine is greater than that which could be obtained from 

 the re-condensation 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. Mr. Smyth informed us in his interesting discourse, that we 

 dig annually 84 millions of tons of coal from our pits. The amount of 

 mechanical force represented by this quantity of coal seems perfectly 

 fabulous. The combustion of a single poun'd of coal, supposing it to 

 take place in a minute, would be equivalent to the work of 300 

 horses; and if we suppose 108 millions of horses working day and 

 night with unimpaired strength, for a year, their united energies would 

 enable them to perform an amount of work just equivalent to that 

 which the annual produce of our coal-fields would be able to accom- 

 plish. 



Comparing the energy of the force with which oxygen and carbon 

 unite together, with ordinary gravity the chemical affinity seems almost 

 infinite. But let us give gravity fair play ; let us permit it to act 



