COMMON THINGS FIRE. 



essence, which with three others, air, water, and earth, con- 

 stituted all natural bodies. 



It was only towards the close of the last century, and within the 

 lifetime of the elder part of the present generation, that the true 

 character of fire was discovered. 



2. It is now known that fire is neither a distinct substance nor 

 essence, as supposed by the ancients. It is a phenomenon con- 

 sisting of the sudden and abundant evolution of heat and light 

 produced when a certain class of bodies called COMBUSTIBLES enter 

 into chemical combination with the oxygen gas which, as has been 

 explained in our Tract on Air, constitutes one of the constituents 

 of the atmosphere. The term COMBUSTION in the modern nomen- 

 clature of physics has been adopted to express this phenomenon. 



3. The class of combustible substances which are commonly 

 used for the production of artificial heat is called FUEL. Such, for 

 example, are pit coal, charcoal, and wood. 



Another class of combustibles is used for the production of 

 artificial light : such, for example, are oil, wax, and the gas ex- 

 tracted from certain sorts of pit coal, from oil, and from certain 

 sorts of wood, such as the pitch pine. 



4. The principal constituents of all these combustibles, whether 

 used for the production of heat or light, are those denominated by 

 chemists CARBON and HYDROGEN. 



CARBON is the name given to charcoal when it is absolutely pure, 

 which it never is as it is obtained by the ordinary industrial processes. 

 It is in that state combined with various heterogeneous and incom- 

 bustible substances. In the laboratories of chemists it is separated 

 from these, and obtained in a state of perfect purity, being there 

 distinguished from the charcoal of commerce by the name CARBON. 



Carbon having never been resolved by any chemical agent into 

 other constituents, is classed in physics as a simple and elementary 

 body, which enters largely into the composition of a most numerous 

 class of bodies which are found in nature, or produced in the 

 processes of industry, the sciences, and the arts. 



5. HYDROGEN has been already very fully described and ex- 

 plained in our Tract upon Water ; we shall presently explain still 

 more in detail its leading properties. Like carbon, it is classed as 

 a simple and elementary substance ; and also, like carbon, enters 

 largely into the composition of a numerous class of bodies. 



6. A quantity of charcoal being placed in a furnace through 

 which a draught of air is maintained, if a part of it be heated to 

 redness, the entire mass will soon become incandescent, and will 

 emit a reddish light, which will be whiter as the air is passed 

 through it more briskly, and will emit considerable heat. The 

 charcoal will gradually decrease in quantity, and at length will 



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