THE CHEMISTRY OP BACTERIA. 119 



shine like the stars in the heavens and when very numerous 

 a diminutive " milky way" will be seen. 



The character of the light emitted by these bacteria 

 varies somewhat for the various species, but not enough to 

 be of use in distinguishing between them. Moreover, the_ 

 light will vary according to the composition of the soil. 

 It may be whitish, bluish or greenish. The intensity of the 

 light, at times, is such that a streak culture, on inclined 

 fish-gelatin, is sufficient to reveal the time, on a watch. In- 

 deed, photographs of a watch, fish, etc., have been made 

 by means of such light. When examining cultures for phos- 

 phorescence it is essential that the eye should be accus- 

 tomed to darkness, in order to perceive the delicate bacterial 

 light. At night time the phosphorescence can be recog- 

 nized, at once, by any one. In the day time, the transition 

 from bright day-light to total darkness is so sudden that it 

 will be necessary to wait for some minutes before the phos- 

 phorescence will be perceived. 



Very little that is positive can be said regarding the 

 mechanism of phosphorescence. The commonly accepted 

 view is that the emission of light is the result of intracel- 

 lular activity. As long as the protoplasm of the cell is at 

 work it liberates energy. This is ordinarily set free as 

 heat, but in these organisms it is supposed that the energy 

 or wave motion is, in part at least, that which corresponds 

 to light. On the other hand, the striking similarity be- 

 tween pigment production and phosphorescence should not 

 be overlooked. Thus, free access of oxygen is essential to 

 both phenomena; high temperatures are unfavorable and 

 even give rise to atypical species or varieties; low temper- 

 atures are especially favorable to the production of pigment 

 and of light. In the latter case, both may remain bright 

 and unaltered after the lapse of many weeks. It would 

 seem, therefore, that phosphorescence, like color, is asso- 

 ciated with the production of certain bacterial products 



