352 ON THE COl^SERVATION" OF FORCE. 



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 holding the atoms of 

 carbon and oxygen firmly united ; they can no longer 

 proiuce 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 car- 

 bonic 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, just as 

 we can restore the potentiality of a weight by raising it 

 from the ground ? It is indeed possible. We shall after- 

 wards 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. Hydrogen 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 combustion of the same weight of carbon. The 

 product of combustion is water, which, therefore, is not of 

 itself further combustible, for the hjdrogen in it is com- 

 pletely saturated with oxygen. The force of affinity, 



