346 PRINCIPLES OF CHEMISTRY 



Many of the hydrocarbons which are met with in nature are the 

 products of organisms, and do not belong to the mineral kingdom. A 

 still greater number are produced artificially. These are formed by what 

 is termed the combination of residues. For instance, if a mixture of 

 the vapours of hydrogen sulphide and carbon bisulphide be passed 

 through a tube in which copper is heated, this latter absorbs the sul- 

 phur from both the compounds, and the liberated carbon and hydrogen 

 combine to form a hydrocarbon, methane. If a mixture of bromo- 

 benzene, C 6 H 5 Br, and ethyl bromide, C. 2 H 5 Br, be heated with metallic 

 sodium, the sodium combines with the bromine of both compounds, 

 forming sodium bromide. From the first combination the group C G H-, 

 remains, and from the second C 2 H 5 . Having an odd number of hydrogen 

 atoms, they in virtue of the law of even numbers cannot exist alone, 

 therefore they combine together forming the compound C 6 H V C 2 H 5 or 

 C 8 H , o (ethylbenzene). Hydrocarbons are also produced by the breaking 

 up of more complex organic or hydrocarbon compounds, especially 

 by heating that is, by dry distillation. For instance, gum- benzoin 

 contains an acid called benzoic acid, C 7 H G O 2 , the vapours of which, 

 when passed through a heated tube, split up into carbonic anhydride, 

 CO 2 , and benzene, C 6 H 6 . Carbon and hydrogen only unite directly in 

 one degree of combination namely, to form acetylene, having the com- 

 position C 2 H 2 , which, as compared with other hydrocarbons, exhibits 

 a very great constancy at a high temperature. 211 . 



density. Where n = l, 2, 8, the hydrocarbons CH 4 , CoH c , C,-;Hg are gases, more and more 

 readily liquefiable. The temperature of the absolute boiling point for CH 4 = 100 3 , and 

 for ethane C.,,H 6 , and in the higher members it rises. The hydrocarbon C,H 10 , already 

 liquefies about 0. C 5 H K > boils at from + 9 (Livoff) to 37, C H U from 58 to 78, Arc. 

 The specific gravities in a liquid state at 15 are : 



0-63 0-66 0'70 0'75 U'5 



29 If, at the ordinary temperature (assuming therefore that the water formed will be 

 in a liquid state), a gram molecule (26 grams) of acetylene, C 2 H.>, be burnt, 310 thousand 

 calories will be emitted (Thomsen), and as 12 grams of charcoal produce 97 thousand 

 calories, and 2 grams of hydrogen 69 thousand calories, therefore, if the hydrogen and 

 carbon of the acetylene were burnt there would be only 2x97 <>9, or 263 thousand 

 calories produced. It is evident, then, that acetylene in its formation absorbs :ilo -it',:;, 

 or 47 thousand calories that is, the reaction of charcoal and hydrogen is endotherinal 

 -and the product is in this respect a substance resembling nitrous oxide, hydrogen 

 peroxide, &c. Such reasoning is, however, open to certain defects of one kind or another, 

 especially when a reaction, which can really only take place at a high temperature, is 

 supposed to be effected at ordinary temperatures or vice versa. Calculations cannot 

 be made for high temperatures, on account of insufficient evidence as to specific 

 heat. 



For considerations referring to the combustion of carbon compounds, we will first 

 enumerate the quantity of heat separated by the combustion of definite eheinical earhon 

 compounds, and then a few figures touching the kinds of fuel applied in a practical 

 way. 



