308 ON THE CONSERVATION OF FORCE. 



When the carbon is burned, we obtain in its stead, and in 

 that of the oxygen, the gaseous product of combustion carbonic 

 acid. Immediately after combustion it is incandescent. When 

 it has afterwards imparted heat to the vicinity, we have in the 

 carbonic acid the entire quantity of carbon and the entire 

 quantity of oxygen, and also the force of affinity quite as strong 

 as before. But the action of the latter is now limited to hold- 

 ing the atoms of carbon and oxygen firmly united ; they can no 

 longer produce either heat or work any more than a fallen weight 

 can do work if it has not been again raised by some extraneous 

 force. When the carbon has been burnt we take no further 

 trouble to retain the carbonic acid ; it can do no more service, 

 we endeavour to get it out of the chimneys of our houses as 

 fast as we can. 



Is it possible, then, to tear asunder the particles of carbonic 

 acid, and give to them once more the capacity of work which 

 they had before they were combined, j ast as we can restore the 

 potentiality of a weight by raising it from the ground 1 It is 

 indeed possible. We shall afterwards see how it occurs in the 

 life of plants; it can also be effected by inorganic processes, 

 though in roundabout ways, the explanation of which would 

 lead us too far from our present course. 



This can, however, be easily and directly shown for another 

 element, hydrogen, which can be burnt just like carbon. Hy- 

 drogen with carbon is a constituent of all combustible vegetable 

 substances, among others, it is also an essential constituent of 

 the gas which is used for lighting our streets and rooms ; in the 

 free state it is also a gas, the lightest of all, and burns when 

 ignited with a feebly luminous blue flame. In this combustion 

 that is, in the chemical combination of hydrogen with oxygen, 

 a very considerable quantity of heat is produced ; for a given 

 weight of hydrogen, four times as much heat as in the combus- 

 tion of the same weight of carbon. The product of combustion 

 is water, which, therefore, is not of itself further combustible, 

 for the hydrogen in it is completely saturated with oxygen. 

 The force of affinity, therefore, of hydrogen for oxygen, like 

 that of carbon for oxygen, performs work in combustion, 



