92 Professor W. Thomson on Mechanical 



bines with the oxygen they inhale. The heat, sound, and 

 mechanical effects, produced by the explosion of gunpowder 

 are, all together, equivalent to the energy of chemical affinity 

 between the different substances of which the unburned pow- 

 der is composed. The potential energy of war is contained in 

 the stores of gunpowder and food which are brought to the 

 field. The gunpowder carried by artillery and infantry con- 

 tains all the potential energy ordinarily brought into action 

 by those two arms of the service. The men's food, and the 

 forage for the horses, contain the stores of potential energy 

 drawn upon in a charge of cavalry. Artillerymen, foot-sol- 

 diers (unless employed to make a bayonet charge), sailors, 

 steamers with their engines, guns, swords, are only means and 

 appliances by which the potential energy contained in the 

 stores of gunpowder and food is directed to strike the blows 

 by which the desired effects are produced. 



The heat and mechanical actions of animals are transforma- 

 tions of the potential energy of their food, mechanically equi- 

 valent to the heat that would be got by burning it. The food 

 of animals is either vegetable, or animal fed on vegetable, or 

 ultimately vegetable after several removes. Now, — except 

 mushrooms and other funguses, which can grow in the dark, 

 are nourished by organic food like animals, and absorb 

 oxygen and exhale carbonic acid, like animals, — all known 

 vegetables get the greater part of their substance, certainly 

 all their combustible matter, from the decomposition of car- 

 bonic acid and water absorbed by them from the air and soil. 

 The separation of carbon and of hydrogen from oxygen in 

 these decompositions is an energetic effect, equivalent to the 

 heat of recombination of those elements by combustion, or 

 otherwise. The beautiful discovery of Priestley, and the sub- 

 sequent researches of Sennebier, De Saussure, Sir Humphrey 

 Davy, and. others, have made it quite certain that those de- 

 compositions of water and carbonic acid only take place 

 naturally in the daytime, and that light falling on the green 

 leaves, either from the Sun or from an artificial source, is an 

 essential condition, without which they are never effected. 

 There cannot be a doubt but that it is the dynamical energy 

 of the luminiferous vibrations which is here efficient in forcing 



