46 THE DATA OF BIOLOGY. 



rapidity of such re-distributions, this raised state of molecular 

 vibration is conspicuous. And we here see that this raised 

 state of molecular vibration, is itself a continuous consequence 

 of the continuous molecular re-distributions it facilitates. 

 The heat generated by each increment of chemical change, 

 makes possible the succeeding increment of chemical change. 

 In the body this connexion of phenomena is the same as we 

 see it to be out of the body. Just as in a burning piece of 

 wood, the heat given out by the portion actually combining 

 with oxygen, raises the adjacent portion to a temperature at 

 which it also can combine with oxygen ; so, in a living 

 animal, the heat produced by oxidation of each portion of 

 tissue, maintains the temperature at which the unoxidized 

 portions can be readily oxidized. 



§ 19. Among the forces called forth from organisms by 

 re-action against the actions to which they are subject, is 

 Light. Phosphorescence is in some few cases displayed by 

 plants — especially by certain fungi. Among animals it is 

 comparatively common. All know that there are several 

 kinds of luminous insects ; and many are familiar with the 

 fact that luminosity is a characteristic of various marine 

 creatures. 



Most of the evidence goes to show that this evolution of 

 light, as well as the evolution of heat, is consequent on oxi- 

 dation of the tissues. Light, like heat, is the expression of a 

 raised state of molecular vibration : the difference between 

 them being a difference in the rates of vibration. Hence by 

 chemical action on substances contained in the organism, heat 

 or light may be produced, according to the character of the 

 resulting molecular vibrations. The inference that 



oxidation is the cause of this luminosity, does not, however, 

 rest only on a priori grounds. It is supported by experi- 

 mental evidence. In phosphorescent insects, the continuance 

 of the light is found to depend on the continuance of respira- 

 tion ; and any exertion which renders respiration more active, 



