QUANTITY DISTINCT FRO&' "iNTEttfeTlY oV HEAT/ J 10 1 



one gramme of carbon, in being oxidised or burned into carbonic 

 anhydride, gives out 8 units of heat, or will raise the temperature 

 of 8 kilogrammes that is to say, 8000 times its own weight of 

 water one degree. 



(107.) It must be remembered, of course, that the quantity of 

 heat evolved during the oxidation of a given weight of hydrogen, 

 carbon, or other combustible, is perfectly independent of the 

 rapidity or slowness of the action. Provided only that the same 

 products are formed, the same amount of heat is liberated in 

 their formation, whether it takes place rapidly or slowly, vio- 

 lently or gradually. It is only the intensity of the heat, and not 

 its quantity, which varies with the rapidity of the combination. 

 When a stout piece of iron rusts in the air, we get oxide of iron 

 produced as the result of the slow burning of the metal, but there 

 is no obvious rise of temperature. On the other hand, when a 

 piece of iron wire is burned in oxygen gas, we have a brilliant 

 combustion, and an intense development of heat. In reality, the 

 products formed in these two cases are not identical, but only 

 allied. Assuming them, however, to be identical, the amount of 

 heat given out in the rapid burning of the metal would be iden- 

 tical with that given out during its slow rusting. The difference 

 is merely that in the one case all the heat is given out in the 

 course of a few seconds that there is a great quantity of heat 

 produced in a short time while in the other case this same 

 quantity of heat is developed only during a long series of years. 

 As a matter of fact, the quantity of heat evolved by the slow 

 rusting of a given weight of iron is considerably greater than 

 that evolved by its rapid combustion in oxygen, the resulting 

 compound being not only more highly oxidised, but in a state of 

 hydration, or combination with solid water. 



(108.) A very ordinary experiment will serve, perhaps, to 

 illustrate more strikingly the point now under consideration. 

 Upon immersing this plate of copper in a jar of chlorine gas, for 

 instance, the chlorine combines gradually with the metal, and 

 there is no evident rise of temperature ; but when very thin 

 copper leaf is immersed in chlorine gas, the combination takes 



