CARBON AND THE HYDROCARBONS 857 



carbons burn, and, according to the amount of carbon they contain, 

 their combustion is attended more or less with a separation of soot 

 that is, finely divided charcoal which imparts great brilliancy to the 

 flame, and on this account many of them are used for the purposes of 

 illumination as, for instance, kerosene, coal gas, oil of turpentine. 

 As hydrocarbons contain reducing elements (that is, those capable of 

 combining with oxygen), they often act as reducing agents as, for 

 instance, when heated with oxide of copper, they burn, forming car- 

 bonic anhydride and water, and leave metallic copper. Gerhardt proved 

 that all hydrocarbons contain an even number of hydrogen atoms. 

 Therefore, the general formula for all hydrocarbons is C rt H 2m where 

 n and m are whole numbers. This fact is known as the law of even 

 numbers. Hence, the simplest possible hydrocarbons ought to t be : 

 CH 2 , CH 4 , CH 6 . . . C 2 H 2 , C 2 H 4 , C 2 H C , C 2 H 8 ... but they do not 

 all exist, since the quantity of H which can combine with a certain 

 amount of carbon is limited, as we shall learn directly. 



Some of the hydrocarbons are capable of combination, whilst others 

 do not show that power. Those which contain less hydrogen belong to 

 the former category, and those which, for a given quantity of carbon, 

 contain the maximum amount of hydrogen, belong to the latter. The 

 composition of those last mentioned is expressed by the general formula 

 C rt H 2u+2 . These so-called saturated hydrocarbons are incapable of 

 combination. 25 The hydrocarbons CH 6 , C 2 H 8 , C 3 H IO , &c. . . do not 

 exist. Those containing the maximum amount of hydrogen will be 

 represented by CH 4 (n = 1, 2n + 2 = 4), C 2 H 6 (7* = 2), C 3 H 8 (n = 3), 

 C 4 Hi , &c. This maybe termed the law of limits. Placing this 1 ' in 

 juxtaposition with the law of even numbers, it is easy to perceive that 

 the possible hydrocarbons can be ranged in series, the terms of which 

 may be expressed by the general formulae C rt H 2n+2 , C rt H 2n , C n H 2tt _ 2 , 

 &c. . . Those hydrocarbons which belong to any one of the series 



weak (1 p.c.) solution of potassium permanganate, KMnO 4 , at ordinary temperatures, 

 they form glycols for example, C 2 H 4 yields C 2 H 6 O 2 . 



25 My article on this subject appeared in the Journal of the St. Petersburg Academy 

 of Sciences in 1861. Up to that time, although many additive combinations with hydro* 

 carbons and their derivatives were known, they "had not been generalised, and were even, 

 continually quoted as cases of substitution. Thus the combination of ethylene, C 2 H 4 , 

 with chlorine, C1 2 , was often regarded as a formation of the products of the substitution 

 of C 2 H 5 C1 and HC1, which it was supposed were held together as the water of crystallisa- 

 tion is in salts. Even earlier than this (1857, Journal of the Petroffsky Academy) I 

 considered similar cases as true compounds. In general, according to the law of limits, 

 an unsaturated hydrocarbon, or its derivative, on combining with rX 2 , gives a substance 

 which is saturated or else approaching the limit. The investigations of Frankland 

 with many organo-metallic compounds clearly showed the limit in the case of metallic 

 compounds, which we shall constantly refer to later on. 



