OPTICS, CHEMISTRY, AND BIOLOGY 65 



compounds of remarkable instability. When, in any 

 aggregation of solid and liquid matter, these com- 

 pounds are continually formed and destroyed, and 

 when the energy thus liberated is directed towards 

 increasing this metabolism and making it more per- 

 manent, we diagnose the presence of organic life. 



Perhaps the nearest physical counterpart to an 

 organism is a flame. In a flame there is a definite 

 chemical change going on whose effect is directed 

 towards its own stability, and is derived from the 

 energy liberated in the change. There is also a 

 constant change of the matter constituting the 

 flame, and even a differentiation of parts, combus- 

 tible matter being taken in and mixed with oxygen 

 below, and products of combustion being eliminated 

 above. There is also growth to a maximum de- 

 pending on the conditions of " nutrition." There is 

 death when the nutrition falls below a certain mini- 

 mum. Flames are capable both of union and fission, 

 and the latter is indeed the common method of 

 propagation. As regards the supply of energy and 

 the final products of its transformation, the flame 

 resembles the animal rather than the vegetable 

 kingdom. But the analogy can be carried much 

 further, though in doing so it tends to become more 

 and more superficial. Thus, in lighting a candle we 

 strike a match. The head of the match is the germ 

 which at a certain temperature bursts into life, and 

 that life feeds for some time on the stick. On 1 



B 



