104 BIOLOGICAL LECTURES. 



In his entry "Sunday, July 10, 1814, Geneva," Faraday de- 

 scribes his experiments on glow-worms. 



"This evening many glow-worms appeared, and of four 

 which I had put in a tumbler with green leaves, two shone 

 very brightly. I separated the luminous part of one in full 

 vigor from the body. It soon faded, and in about ten minutes 

 ceased to emit light ; but on pressing it with a knife, so as to 

 force the matter ottt of the skin, it again became luminous, and 

 continued to shine for two hours brightly. One I found on the 

 floor crushed unawares by the foot. I separated the luminous 

 part of this insect, and left it on paper. It shone with undi- 

 minished luster the whole evening, and appeared not at all to 

 have suffered in its power of emitting light by the mixture and 

 confusion of its parts, so that it appears to depend more upon the 

 chemical nature of the substance than upon the vital powers of 

 the animal ; but at the same time, it appears, from the variations 

 in splendor, accompanied by motions in the living animal, that it 

 may be much influenced or modified by, or in some manner 

 submitted to, the powers of the worm. 



"The matter which appears to fill the hinder part of the 

 body in the shining season is yellowish-white, soft, and glutinous. 

 It is insoluble, apparently, in water or in alcohol. It does not 

 immediately lose its power of shining in water. Heat forces 

 out a bright glow, and then it becomes extinct ; but if not 

 carried too far, the addition of moisture after a time revives its 

 power. No motion or mixture seems to destroy its power 

 whilst it remains fresh and moist, but yet a portion thus rubbed 

 sooner lost its light than a portion left untouched. The time 

 of its continuance in a luminous state was very various, and 

 perhaps depends upon the state of the worms from which it 

 was taken. The death of the worm seemed to have no imme- 

 diate effect upon the illumination of the hinder part ; and with 

 respect to the length of time that it continued to shine after- 

 wards, it seemed indifferent whether it was left on the body or 

 taken off; but when extinct, exposure of the interior to air 

 always caused a fresh emanation of light. I found a worm 

 which emitted light from a very small part of the body, and 

 very feebly, and for a very short time together. The worm 



