360 PRINCIPLES OF CHEMISTRY 



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

 iodine, fuming sulphuric acid, or sulphuric anhydride. If olefiant gas 

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

 and be subjected to continued shaking (as, for instance, by suspending 

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

 mixing causes the olefiant gas, little by little, to combine with the 

 sulphuric acid, forming C 2 H 4 H. 2 SO 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 2 O = C 2 H 6 O. In this reaction (Ber- 

 thelot) we see an excellent example of the fact that if a given sub- 

 stance, 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. 

 Therefore the reaction, CH 3 CH 2 (OH)=H(OH) +CH 2 CH,, ought 

 to be classed as a reversible reaction. In combination with various 

 molecules, X 2 , ethylene gives saturated compounds, C 2 H 4 X 2 that is, 

 either CH 2 XCH 2 X, or CH 3 CHX 2 , which correspond with ethane, 

 CH 3 CH 3 , or C 2 H~. 48 



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

 Berthelot (1857). It has a very pungent smell, is characterised 

 by its great constancy under the action of heat, and is the only 

 product of the direct combination of carbon with hydrogen. This 

 combination takes place under the action of great heat, such as is 

 produced by a very strong electric current between carbon points. A 

 brilliant arc (voltaic) is then formed between the carbon electrodes, 

 which 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 . 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 hydro- 

 bromic acid is taken 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 of hydrobromic acid is withdrawn by passing it through anhydrous 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 cir- 



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

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

 can be expressed thus : (CH^CH^);/ (CH) 2 C r 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 '6x + z, whence z 4- 2r = x ; by 

 this means the possible isomerides are determined. For example, for butylenes, 

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



