CAfcBON AND THE HYDROCARBONS 859 



the combination of residues. For instance, if a mixture of the vapours 

 of hydrogen sulphide arid carbon bisulphide be passed through a tube 

 in which copper is heated, this latter absorbs the sulphur from both 

 the compounds, and the liberated carbon and hydrogen combine to 

 form a hydrocarbon, methane. If carbon be combined with any metal 

 und this compound MC n be treated .with an acid HX, then the 

 haloid X will give a salt with the metal and the residual carbon and 

 hydrogen will give a hydrocarbon. Thus cast iron which contains a 

 compound of iron and carbon gives liquid hydrocarbons like naphtha 

 'under the action of acids. 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, NaBr. From the first combination the group C 6 H 5 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, and there- 

 fore combine together forming the compound C 6 H5.C 2 H 5 or C 8 H, 

 (ethylbenzene). Hydrocarbons are also produced by the breaking up 

 of more complex organic or hydrocarbon compounds, especially by heat- 

 ingthat is, by dry distillation. For instance, gum-benzoin contains 

 :an acid called benzoicacid, C 7 H 6 O 2 , the vapours of which, when passed 

 through a heated tube, split up into carbonic anhydride, CO 2 , and 

 benzene, C 6 H C . Carbon and hydrogen only unite directly in one ratio 

 >of combination namely, to form acetylene, having the composition 

 C 2 H 2 , which, as compared with other hydrocarbons, exhibits a very 

 great stability at a somewhat high temperature. 29 



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

 in a liquid state) a gram molecule (26 grams) of acetylene, CaH 2 , be burnt, 810 thousand 

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

 calories, and 2 grams of hydrogen 69 thousand calories, it follows that, if the hydrogen 

 and carbon of the acetylene were burnt there would be only 2x97f69, or 265 

 thousand calories produced. It is evident, then, that acetylene in its formation absorbs 

 810-268, Or 47 thousand calories. 



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

 enumerate the quantity of heat separated by the combustion of definite chemical carbon 

 compounds, and then give a few figures bearing on the kinds of fuel used in practice. 



For molecular quantities in perfect combustion the following amounts of heat are 

 given out (when gaseous carbonic anhydride and liquid water are formed), according to 

 Thomson's data (1) for gaseous CnHjn+s: 52 < 8H-158'8tt. thousand calories; (2) for 

 C,iH 2n : 17'7 + 158'ln thousand calories ; (8) according to Stohmann (1888) for liquid 

 saturated alcohols, C n H 2n 4. 2 O : II' 8 + 156<8n > an <* as tb - e latent heat of evaporation = about 

 8'2 + 0'en, in a gaseous state, 20'0 + 156'9n; (4) for monobasic saturated liquid acids, 

 C n H 2 n0 2 : 95'8 + 154-3, and as their latent heat of evaporation is about 5'0 + l-2w, in a 

 gaseous form, about 90 + 155n; (5) for solid saturated bibasic acids, C n H 2n _ 2 O 4 : 

 253'8 + 152-6w, if they^are expressed as C n H 2 nC 2 H 2 O 4 , then 51'4 + 152'6n ; (6) for ben- 

 zene and its liquid homologues (still according to Stohmann) G n H 2n _ f) : 158-6 + 156-3tt, 

 and in a gaseous form about 155 + 157^; (7) for the gaseous homologues of acetylene, 

 -jj (according to Thomsen) 5 + 157w, It is evident .from the preceding figures 



