GLOW-WORM. 141 



worm is a winged insect, which flies low in the 

 evenings but emits no light. 



THE GLOW-WORM, MALE AND FEMALE. 



But we must just notice one or two of those 

 effects of heat which are not so obviously connected 

 with the display of light ; for though we should 

 continue to write, and read, and observe, till the 

 light of our own eyes were extinguished, we should 

 be no more near the end of the beautiful subject of 

 light than we are at this moment. 



The most general and most active property of 

 heat, is that of overcoming the cohesion of the 

 parts of substances ; and thus softening them, and 

 expanding them into more bulk or space. It 

 acts with very different degrees both of rapidity 

 and of energy in different substances; but it is 

 probable that there is no substance that could not 

 be melted, and after that changed into air or va- 

 pour, by a sufficient degree of heat applied under 



