74 ANIMAL CHEMISTRY LECTURE IV. 



as in the heating of platinum wire, so much the more external 

 work done, so much the less internal heat developed. By the 

 electrolytic decomposition of water, the heat of the burning zinc, 

 which does not appear internally in the cell, and which might be 

 manifested externally in the ignition of platinum wire, lies dor- 

 mant in the separated oxygen and hydrogen ; but it is not lost. 

 On the contrary, I can render it evident to you with the greatest 

 ease. Now that we have collected a sufficient quantity of our 

 electrolytic gases, I have only to mix them together and explode 

 them, when you observe a considerable evolution of light and 

 heat resulting from their combination ; which light and heat are 

 nothing more than the light and heat of the burning zinc, not 

 manifested in the cell, but retained for a time in the separated 

 gases, so as to constitute their potential energy. The explosion 

 of the gases at once or a hundred years hence would make no 

 difference. The heat and light resulting from their eventual 

 explosion would still be the heat and light of the burning zinc 

 stored up in them, so to speak, at the moment of their electro- 

 lysis. In my last lecture I showed you the formation of water 

 by the combustion of hydrogen in the oxygen of the air, and 

 called your attention specially to the heat developed by the com- 

 bination. The heat so developed did not originate in the oxygen 

 and hydrogen, but was simply a liberation of the heat force, 

 which, directly or indirectly, at some time or other, had been 

 employed in pulling them apart, and been rendered latent in 

 them so long as they continued apart just as the force of a 

 stretched cross-bow does not originate in the bow, but is merely 

 latent muscular force stored up in the separated bow and cord. 

 Similarly in the brilliant combustion of carbon in oxygen gas to 

 form carbonic anhydride, there was no generation of heat, but 

 only a liberation of the heat previously stored up in the two 

 separated elements. 



(79.) At length, then, we are in a position to understand the 

 nature of the action taking place in the vegetable kingdom, by 

 which carbonic anhydride and water are decomposed to con- 

 sider what is the external force employed in the pulling apart of 



