372 PRINCIPLES OF CHEMISTRY 



Like other unsaturated hydrocarbons, olefiant gas readily enters 

 into combination with certain substances, such as chlorine, bromine, 

 iodine, fuming sulphuric acid, or sulphuric anhydride, &c. If defiant 

 gas be sealed up with a small quantity of sulphuric acid in a glass 

 vessel, and constantly agitated (as, for instance, by attaching it to the 

 moving part of a machine), the prolonged contact and repeated mixing 

 causes the defiant gas, little by little, to combine with the sulphuric 

 acid, forming C 2 H 4 H 2 S0 4 . If, after this absorption, the sulphuric acid 

 be diluted with water and distilled, alcohol separates, which is produced 

 in this case by the olefiant gas combining with the elements of water, 

 C 2 H 4 -h H 2 O = C 2 H 6 O. In this reaction (Berthelot) we see an excellent 

 example of the fact that if a given substance, like olefiant gas, is produced 

 by the decomposition of another, then in the reverse way this substance, 

 entering into combination, is capable of forming the original substance 

 in our -example, alcohol. In combination with various molecules, 

 X 2 , ethylene gives saturated compounds, C 2 H 4 X 2 or CH 2 XCH 2 X 

 (for example, C 2 H 4 C1 2 ), which correspond with ethane, CH 3 CH 3 or 

 C,H 6 



Acetylene, C 2 H 2 = CHCH, is a gas ; it was first prepared by Ber- 

 thelot (1857). It has a very pungent smell, is characterised by its 

 great stability under the action of heat, and is obtained as the only 

 product of the direct combination of carbon with hydrogen when a 

 luminous arc (voltaic) is formed between carbon electrodes. This arc 

 contains particles of carbon passing from one pole to the other. If the 

 carbons be surrounded with an atmosphere of hydrogen, the carbon in 

 part combines with the hydrogen, forming C 2 H 2 . 48 bis Acetylene may 

 be formed from olefiant gas if two atoms of hydrogen be taken 

 from it. This may be effected in the following way : the olefiant gas is 

 first made to combine with bromine, giving C 2 H 4 Br 2 ; from this the 

 hydrobromic acid is removed by means of an alcoholic solution of 

 caustic potash, leaving the volatile product C 2 H 3 Br ; and from this 

 yet another part of hydrobromic acid is withdrawn by passing it through 

 anhydrous alcohol in which metallic sodium has been dissolved, or by 

 heating it with a strong alcoholic solution of caustic potash. Under 

 these circumstances (Berthelot, Sawitsch, Miasnikoff) the alkali takes 

 up the hydrobromic acid from C /1 H 2u _,Br, forming C 1| H 2// _ 2 . 



48 The homologues of ethylene, C n H 2n , are also capable of direct combination with 

 halogens, &c., but with various degrees of facility. The composition of these homologuea 

 can be expressed thus : (CH 3 )*(CH. 2 )j, (CH) 2 C,-, where the sum of x + z is always an even 

 number, and the sum of x + z + r is equal to half the sum of Sa; + 2, whence z + 2r = x ; by 

 this means the possible isomerides are determined. For example, for butylenes, C 4 H 8l 

 (CH S ) 2 (CH) 2 , (CH 3 ) 2 (CH 2 )C, (CHa) (CH 2 ) 2 CH, and (CH 2 ) 4 are possible. 



48 bis $ ee also method of preparing C 2 H 3 in Note 12 bis. 



