114 SIMPLE AND COMPOUND SUBSTANCES. CARBONIC ACID. 



any known chemical process, be shown to consist of others united 

 together. Such, to take familiar examples, are the various 

 metals, with sulphur, iodine, as well as many other less common 

 bodies. But the greater part of the substances which surround 

 us 7 are termed compound; because they can be separated into 

 two or more simple or elementary bodies. Limestone, for ex- 

 ample, when exposed to heat, is much changed in its character ; 

 it gives off a kind of gas or air, termed carbonic acid gas ; and 

 lime remains a substance which it is no longer safe to handle, 

 on account of its possessing the power of destroying (or, as com- 

 monly said, burning) animal flesh, whence it is commonly termed 

 quick-lime. But neither of these two substances are simple ; 

 for it is easily shown, that carbonic acid is composed of two 

 others, of which one is carbon, a solid substance of which the 

 diamond is the purest form, but which is nearly the same with 

 charcoal ; whilst the other is oxygen, a gas which forms part of 

 the air we breathe. Again the lime may be shown (by a pro- 

 cess of much difficulty) to consist of a metal, termed calcium, 

 united with some of this same oxygen. 



158. Carbonic acid is the gas known as fixed air, or in 

 mines as choke damp. It is very injurious to the life of animals, 

 acting as a sort of poison to them. It is formed during the 

 combustion or burning of every substance containing carbon ; for 

 this combustion really consists of the union of carbon with 

 oxygen, which, when it takes place rapidly, is accompanied by 

 light and heat. Thus, if a pan of charcoal be burned in a closed 

 room, a large proportion of the oxygen of the air will be con- 

 verted into carbonic acid ; so that any human being, and almost 

 any air-breathing animal, would rapidly lose his life in such an 

 atmosphere. In this manner many persons have been suffocated. 

 Charcoal, however, is not the only form in which carbon exists ; 

 coal contains a very large proportion of the same element ; and 

 carbonic acid is accordingly formed by its combustion. It also 

 exists in the gas now so commonly used in towns for lighting, 

 which is usually made from coal, though sometimes from oil ; 

 this gas consists of carbon in union with hydrogen, another kind 

 of element presently to be noticed ; and when it is burned it 



