CARBON AND THE HYDROCARBONS 865 



Tolerably pure marsh gas 35 may be obtained by heating a mixture of an 

 acetate with an alkali. Acetic acid, C 2 H 4 O 2 , on being heated is decom- 

 posed into marsh gas and carbonic anhydride, C 2 H 4 2 =CH 4 -t-C0 2 . 



An alkali for instance, NaHO gives with acetic acid a salt, 

 C 2 H 3 Na0 2 , which on decomposition retains carbonic anhydride, form* 

 ing a carbonate, ]S"a 2 C0 3 , and marsh gas is given off : 



C 2 H 3 Na0 2 + NaHO=Na 2 C0 3 + CH 4 



Marsh gas is difficult to liquefy ; it is almost insoluble in water, 

 and is without taste or smell. The most important point in connection 

 with its chemical reactions is that it does not combine directly with 

 anything, whilst the other hydrocarbons which contain less hydrogen 

 than expressed by the formula C n H 2u+2 are capable of combining with 

 hydrogen, chlorine, certain acids, &c. 



If the law of substitution gives a very simple explanation of the 

 formation of hydrogen peroxide as a compound containing two- 

 aqueous residues (OH) (OH), then on the basis of this law all hydro- 

 carbons ought to be derived from methane, CH 4 , as being the simplest 

 hydrocarbon. 36 The increase in complexity of a molecule of methane 

 is brought about by the faculty of mutual combination which exists in 

 the atoms of carbon, and, as a consequence of the most detailed study 

 of the subject, much that might have been foreseen and conjectured 

 from the law of substitution has been actually brought about in such 

 a manner as might have been predicted, and although this subject- 

 on account of its magnitude really belongs, as has been already stated, 

 to the sphere of organic chemistry, it has been alluded to here in order 

 to show, although only in part, the best investigated example of the 

 application of the law of substitution. According to this law, a mole- 

 cule of methane, CH 4 , is capable of undergoing substitution in the four 

 following ways : (1) Methyl substitution, when the radicle, equivalent 

 to hydrogen, called methyl CH 3 , replaces hydrogen. In CH 4 this 

 radicle is combined with H and therefore can replace it, as (OH) 

 replaces H because with it it gives water ; (2) methylene substitution, 

 or the exchange between H 2 and CH 2 (this radicle is called methylene), 

 is founded on a similar division of the molecule CH 4 into two equiva,- 



oonducted in metallic pipes to works hundreds of miles distant, principally for metal- 

 lurgical purposes. 



55 The purest gas is prepared by mixing the liquid substance called zinc methyl, 

 Zn(CH 3 ) 2 , with water, when the following reaction occurs : 



Zh(CH 3 ) 2 + 2HOH = Zn(HO) 2 + 2CH S H. 



50 Methylene, CH 2 , does not exist. When attempts are made to obtain it (for 

 example, by removing X 2 from CH 2 X 3 ), G^SL^ or C 3 H 6 are producedthat is to say, it 

 undergoes polymerisation. 



