FOR EXCITING LIGHT. 207 



lations, strongly resembling those produced by an 

 electric discharge through a tube spotted with tin- 

 foil ; lasting but an instant, but capable of being 

 repeatedly excited on irritating the animal. 



" In the glow-worm the luminous matter con- 

 sists of little granules, and is contained in minute 

 sacs covered with a transparent horny lid." 



These sacs are mostly composed of a close net- 

 work of finely divided tracheae, which also ramify 

 through every part of the granular substance. 

 The lid exhibits a number of flattened surfaces, 

 so contrived as to diffuse light in the most advan- 

 tageous manner. 



That electric currents are excited and trans- 

 mitted from one part to another of other animals 

 than electric fishes was long ago incontroverti- 

 bly demonstrated. The life -power both of plants 

 and animals modifies the transmission of electro- 

 dynamic action. All that has been said of the 

 effects of vegetation in producing a disturb- 

 ance of electric equilibrium will apply equally to 

 the nutritive and other processes of animals. 

 M. du Bois-Reymond, in his researches, has proved 

 that there are no two parts of the body, except 

 those which correspond on the opposite sides, 

 whose electric condition is precisely the same ; 

 and that the differences between them are greater, 

 in proportion to the diversity of the vital processes 

 which are taking place in them, and the activity 

 with which these are carried on. Donne says 



