ELECTRICITY. 35 



Some substances when heated to a certain de- 

 gree become luminous ; iron, for instance ; and 

 this is what is called a red heat. 



If bodies heated to redness be introduced into a 

 gas, it does not become visible, and hence it has 

 been concluded that gas is not capable of being 

 made luminous : but it is now considered that 

 flame is hydrogen gas in a luminous state. 



Light is also produced by percussion ; as in the 

 case of a flint and steel. The spark produced in 

 this case is owing to the flint breaking off a small 

 fragment of the steel, which is thus rendered red 

 hot, and burns diu'ing its passage through the air. 

 But two pieces of quartz struck smartly toge- 

 ther also give out light, although here there can 

 be no combustion. 



Instruments for measuring the degree or inten- 

 sity of light are called photometers, 



ELECTRICITY. 



Electricity and galvanism have been already 

 treated of in the first volume. The electric fluid 

 is now considered as a chemical agent of great 

 importance, exciting a powerful influence in the 

 decomposition of bodies. The connexion between 

 electricity and chemical decomposition was first 

 shown by Sir Humphry Davy, to whom the world 

 is indebted for so many brilliant discoveries. 



There is still, however, great uncertainty and 

 various opinions with respect to the real nature of 

 this influence, which is usually classed among the 

 imponderable elementary bodies. 



D 2 



