90 MICROBES AND TOXINS 



of sea-beasts frequently emit light. Butcher-meat left to 

 itself in a cool place for two or three days half-immersed 

 in 3 per cent, salt solution very often becomes luminous. 

 Dead leaves fallen in the forests occasionally give out a 

 dim, steady light. Now it is not the animal or vegetable 

 tissue which shines ; it must be microbes, moulds, or 

 bacteria. 



Luminous microbes have been discovered in the Baltic Sea, 

 the North Sea and the Indian Ocean, and phosphorescent 

 bacteria in the Elbe. At present about fifteen moulds and 

 about thirty bacteria are known to be " photogenic " or 

 " luminous." 



A simple recipe may be given ; take a fresh herring, sprinkle 

 it with 3 per cent, salt solution, leave it at a temperature ot 

 about io°, add a little sugar, glycerine, and peptone; in two 

 days the flesh and the juice become luminous. 



The production of light depends on the temperature and the 

 food supply. Sometimes a temperature from 20 to 30° C. 

 suits best ; most often lower temperatures are more favourable. 

 The phenomenon has been seen to occur at -1- 45° C., and at 

 - 20° C. The luminous bacteria seem to like salt ; some only 

 require a nitrogenous medium, others require in addition 

 carbon. But the indispensable substance is oxygen, and when 

 this is exhausted the luminosity ceases. If a bubble of air is 

 made to pass, by turning upside down, through a long tube in 

 which there is a culture of a luminous bacterium, which has 

 just become extinguished for lack of oxygen, a wave of light 

 can be seen passing along the tube. There is no luminosity in 

 a vacuum. 



Strains have been produced by natural selection so luminous 

 that their light can be seen in full daylight. If a flask coated 

 on the inside with gelatine is inoculated with the Bacterium 

 phosphoreum or the Pseudomonas lucifera, one gets a microbial 

 lamp which with an eye a little accustomed to darkness allows 

 one to read the time from a watch or to read moderately large 

 print. It might be possible even, it seems, to employ such 

 lamps in powder-magazines or mines, for they do not emit heat. 



