THE HYDROCARBONS 147 



carbon and hydrogen and their compounds. The "heat of combustion" 

 of hydrogen is much greater than that of carbon, and the amount of heat 

 produced by combustion of hydrocarbons will therefore vary with the 

 proportion of hydrogen. The "pyrometric effect," that is, the highest 

 temperature attainable by burning the fuel, will, however, largely depend 

 upon the heat absorbed by the gaseous products, and is found to be greater 

 for carbon than for hydrogen. The oxyhydrogen flame is intensely hot, 

 partly because, being fed within with oxygen, combustion of the 

 hydrogen takes place in a very small space. The Bunsen and blow-pipe 

 flames are hot for the same reason. For furnaces where a very high 

 temperature is required, coke or anthracite, which burn with very little 

 flame, are employed. These relations are shown in the following table, 

 in which the heat of combustion of a gram of substance is compared with 

 the temperature attainable when the substance is used as fuel. A "heat 

 unit " is the heat that would raise a gram of water 1° C. 



Fuel. . Heat of Combustion. . ^y''^'"^*"^ ^f ^^l'. 



In Oxygen. In Air. 



It is noticeable that water, both free and combined, diminishes heat 

 production, because to eliminate, vaporise, and raise it in temperature, heat 

 must be absorbed. Partly for the same reason, the heating effect produced 

 by combustion in air is less than by combustion in oxygen, because the 

 nitrogen of the air absorbs heat. 



In the foregoing study, attention has been drawn to compounds 

 that apparently form the starting-points of several series of hydrocarbons, 

 the members of each of which differ from each other by Q\\<^. The series 

 of which methane (marsh gas) CH4, ethane CsHg, and propane CsHg, 

 form the first members, is called the "paraffin series" of hydrocarbons ; 

 the members may be represented by the general formula CnH2n-i-2' 

 Ethylene (olefiant gas) C2H4, and propylene CsIIg, form the first 

 members of a second series, the " olefines," having the general formula 

 CnHgn. Acetylene C2H2, a gas of very high illuminating power, is the 

 first member of a series CnH2n-2- The "terpenes" occurring in the oil 

 of turpentine, obtained by distilling the resinous exudation of certain 



