518 Br. D. Mendeleeff [May 31, 



therefore Cn2(0H)^ should yield, as it actually does, immediately 

 water and the oxide of methylene, CH""0, which is methane with 

 oxygen substituted for two atoms of hydrogen. Exactly in the 

 same manner out of CH(OH)^ are fo/med water and formic acid, 

 CHO(OH), and out of C(OII)^ is produced water and carbonic 

 acid, or directly carbonic anhydride, CO'-, which will tlierefore be 

 nothing else than methane with the double replacement of pairs of 

 hydrogen by oxygen. As nothing leads to the supposition that the 

 four atoms of hydrogen in methane differ one from the other, so it docs 

 not matter by what means we obtain any one of the combinations 

 indicated — they will be identical ; that is to say, there will be no case 

 of actual isomerism, although there may easily be such cases of 

 isomerism as have been distinguished by the term metamerism. 



Formic acid, for example, has two atoms of hydrogen, one attached 

 to the carbon left from the methane, and the other attached to the 

 oxygen which has entered in the form of hydroxyl, and if one of them 

 be replaced by some substance X it is evident that we shall obtain 

 bodies of the same composition, but of different construction, or of 

 different orders of movement among the molecules, and therefore 

 endowed with other properties and reactions. If X be methyl, CH^, 

 that is to say, a group capable of rejdacing hydrogen because it is 

 actually contained with hydrogen in methane itself, then by substi- 

 tuting this group for the original hydrogen, we obtain acetic acid, 

 CCH^O(OH), out of formic, and by substitution of the hydrogen in 

 its oxide or hydroxyl we obtain methyl formiate, CHO(OCH^). These 

 bodies differ so much from each other physically and chemically 

 that, at first sight, it is hardly possible to admit that they contain the 

 same atoms in identically the same proportions. Acetic acid, for 

 example, boils at a higher temperature than water, and has a higher 

 specific gravity than it, while its metamer, formo-methylic ether, is 

 lighter than water, and boils at 30", that is to say, it evaporates very 

 easily. 



Let us now turn to carbon compounds containing two atoms of 

 carbon to the molecule, as in acetic acid, and proceed to evolve 

 them from methane by the principle of substitution. This principle 

 declares at once that methane can only be split up in the four 

 following ways : — 



1. Into a group CH^ equivalent with H. Let us call changes of 

 this nature methylation. 



2. Into a group CH- and H'-. We will call this order of 

 substitutions methylenation. 



3. Into CH and 11^, which commutations we will call acetylena- 

 tion. 



4. Into C and H"^, which may be called carbonisation. 



It is evident that hydrocarbon compounds containing two atoms of 

 carbon, can only proceed from methane, CH*, which contains four atoms 

 of hydrogen by the first three methods of substitution ; carbonising 

 would yield free carbon if it could take place directly, and if the 



