200 Chemistry. [Lecture 31. 



surface. If a ball of sulphur is suddenly plunged 

 into the flame, it will appear that a considerable 

 part of the vapour is not on fire. The flame is 

 a hollow cone, surrounding an entire cone of 

 vapour. 



Flame vibrates, because the vapour is thrown 

 out irregularly from the body, and because, 

 being an elastic fluid, the smallest disturbance 

 in the air makes the two springs, i. e. of the air 

 and of the vapour, act on each other, and pro- 

 duce expansion and contraction. 



Soot is a part of the burning vapour which 

 has not been so much inflamed as the rest. In 

 large flames, the surface on which the air acts is 

 less in proportion to the quantity of vapour than 

 in small flames; hence, proportionally, more 

 smoke and soot is formed from large than small 

 flames. 



Combustibles are judiciously divided into sim- 

 ple and compound. 1. The simple are "hydrogen, 

 phosphorus, sulphur, and carbon. Dr. Thomson 

 observes that the metals might be included, but 

 the greater number of their properties are so 

 different from the four bodies just mentioned, 

 that it is more proper to consider them as a 

 distinct class of bodies. This judicious phi- 

 losopher adds : " All our classifications are in 

 fact artificial ; Nature does not know them, and 

 will not submit to them. They are useful, how- 

 ever, as they enable us to learn a science sooner, 

 and to remember it better : but if we mean to 



