APPENDIX 331 



light are generated the moment steel is knocked against a flint, 

 so the blow of the particles of oxygen of the air against the 

 particles of carbon and hydrogen in coal-gas generates the 

 heat and light that we observe in the gas flame. The only 

 difference between the two processes consists in that in the first 

 case we see the motion, the blow, as well as the accompanying 

 phenomena, i.e. light and heat ; whereas in the latter case we see 

 only these phenomena, and get an idea of the collision of particles 

 only from the results. Before combustion we have a hydrocarbon 

 {i.e. a compound of carbon and hydrogen), the coal-gas, and 

 oxygen, and after combustion carbonic acid and water. 



Therefore every atom of carbon and hydrogen stands to 

 oxygen in the same relation as the ball C stands to the ball 

 O^. Like the balls they are in a state of tension, and possess 

 a store of latent potential energy which we call chemical affinity 

 or chemical attraction. In the separated atoms of carbon and 

 hydrogen we have a fresh illustration of the potential energy 

 of position, which at the moment of the collision of atoms in 

 combustion passes into the energy of motion, i.e. into heat 

 and light. 



This state of tension in the atoms of carbon, this tendency 

 of theirs to unite with the atoms of oxygen, does not attract 

 our attention in daily life, because an impulse is necessary to 

 produce their combination. In order to burn a piece of coal 

 we must set fire to it, i.e. the process of combustion must be 

 initiated from without. This tendency of carbon to combine 

 with oxygen is, however, manifested more obviously in phenomena 

 of spontaneous combustion. It has long been known, for in- 

 stance, that rotting hay in stacks is capable of taking fire spon- 

 taneously, but a case of it has only recently been investigated 

 in Germany. When some large hay-stacks were opened in con- 

 sequence of spontaneous combustion being indicated by the 

 smoke coming out of them, it appeared that the hay inside was 

 already charred, and that the soft, shiny, graphite-like carbon 

 caught fire directly it came into contact with the air. It appeared 

 later on that such spontaneously combustible carbon could be 

 prepared artificially if hay was charred in the absence of air ; 

 in a sealed glass-tube, for instance. Carbon prepared in this 

 way catches fire the moment it comes into contact with the air. 

 This and similar examples obviously prove that combustion, 



