Chap. XXXIV.] SOUXCES OF HEAT. 455 



them, the velocity with which they strike each other. 

 That this velocity is enormous is proved by the 

 amount of heat which it generates/ 5 " When the 

 atoms clash they recoil, and the subsequent tremulous 

 motion is one form of heat." When the heat pro- 

 duced by the combination is sufficiently great, light 

 is also produced, and combustion occurs. In most 

 cases of combustion the chemical combination is of 

 some substance with the oxygen of the air, and as 

 the affinity for oxygen is great so is the heat pro- 

 duced intense. 



If heat is evolved when two bodies enter into 

 combination, heat ought to disappear when the bodies 

 are severed from their combination, when decom- 

 position [ is effected. This is the case : the heat lost 

 in chemical decomposition is exactly equal to that 

 generated by combination. 



The amount of heat produced by the combination 

 of a substance with oxygen is always constant. No 

 matter whether the body be oxydised immediately, or 

 in various stages, the total amount of heat disengaged, 

 till the process is complete, is always the same. Thus 

 a given amount of carbon will always unite with a 

 definite amount of oxygen to produce carbonic acid gas, 

 and will evolve the same amount of heat whether the 

 process be sudden or slow. So it is possible to deter- 

 mine once for all the amount of heat produced by the 

 oxydation of certain quantities of given substances. 

 Thus, the heat generated by the combustion of 1 

 pound of hydrogen would raise the temperature of 

 34-, 46 2 pounds of water 1 C. ; 1 pound of wood 

 charcoal would raise the temperature of 8,080 pounds 

 of water 1 C., and oxide of carbon, 2,403 pounds. 

 The following table of Favre, Silbermann, and 

 Frankland shows the amount of heat (in heat units) 

 disengaged by the oxydation of 1 pound of various 

 food stuffs, etc. The substance was first dried and 



