HEAT AND ATOMIC AFFINITY 



This, in the union of carbon, hydrogen or the com- 

 pounds thereof with oxygen or atmospheric air, con- 

 stitutes the phenomenon of ordinary combustion. The 

 respiration of animals is of the same nature. The 

 carbon and hydrogen constituents of the blood are 

 brought in contact in the lungs with oxygen and con- 

 verted into Carbon-Dioxide and water. Heat being 

 evolved to the same extent, but not with the same 

 intensity as in ordinary combustion. When the mo- 

 lecules of the one body are surrounded by, but not 

 mixed with, the molecules of the other, the chemical 

 action being limited to the surface only of one of the 

 bodies, the change in combination is gradual, pro- 

 ceeds slowly and without violence. If, on the con- 

 trary, such molecules are intimately mixed together, 

 as coal gas when mixed with air, or if oxygen is held 

 in such a combination that a slight rise in tempera- 

 ture will set it free, as it is in the saltpetre contained 

 in gunpowder, the chemical action is transmitted al- 

 most instantly throughout the mass, causing a violent 

 and destructive explosion, with intense heat and 

 consequent expansion of the gases formed, into many 

 hundred times the bulk of the pre-existent masses. 



The many myriads of substances solid, liquid or 

 gaseous, mineral or organic, inanimate or having 

 life that are met with in this world, and we have 

 reason to believe in the thousands and millions of 



