THE HALOGENS 473 



subjected to a further metalepsical substitution the second atom of 

 hydrogen may be substituted by chlorine, and a liquid substance, 

 CH 2 C1 2 , called methylene chloride, will be obtained. In the same 

 manner the substitution may be carried on still further, and CHC1 3 , 

 or chloroform, and lastly carbon tetrachloride, CC1 4 , will be produced. 

 Of these substances the best known is chloroform, owing to its being 

 formed from many organic substances (by the action of bleaching 

 powder) and to its being used in medicine as an anaesthetic ; chloroform 

 boils at 62 and carbon tetrachloride at 78. They are both colourless 

 odoriferous liquids, heavier than water. The progressive substitution 

 of hydrogen by chlorine is thus evident, and it can be clearly seen that 

 the double decompositions are accomplished between molecular quanti- 

 ties of the substance that is, between equal volumes in a gaseous state. 



Carbon tetrachloride, which is obtained by the metalepsis of marsh 

 gas, cannot be obtained directly from chlorine and carbon, but it may be 

 obtained from certain compounds of carbon for instance, from carbon 

 bisulphide if its vapour mixed with chlorine be passed through a 

 red-hot tube. Both the sulphur and carbon then combine with the 

 chlorine. It is evident that by ultimate metalepsis a corresponding 

 carbon chloride may be obtained from any hydrocarbon indeed, the 

 number of chlorides of carbon C ft Cl 2m already known is very large. 



As a rule, the fundamental chemical characters of hydrocarbons are 

 not changed by metalepsis ; that is, if a neutral substance be taken, then 

 the product of metalepsis is also a neutral substance, or if an acid be 

 taken the product of metalepsis also has acid properties. Even the 

 crystalline form not unfrequently remains unaltered after metalepsis. 

 The metalepsis of acetic acid, CH 3 -COOH, is historically the most 

 important. It contains three of the atoms of the hydrogen of marsh 

 gas, the fourth being replaced by carboxyl, and therefore by the action 

 of chlorine it gives three products of metalepsis (according to the amount 

 of the chlorine and conditions under which the reaction takes place), 

 mono-, di-, and tri-chloracetic acids CH 2 C1-COOH, CHC1 2 'COOH, and 

 CC1 3 -COOH ; they are all, like acetic acid, monobasic. The resulting 

 products of metalepsis, in containing an element which so easily acts 

 on metals as chlorine, possess the possibility of attaining a further com? 

 plexity of molecules of which the original hydrocarbon is often in no 

 way capable. Thus on treating with an alkali (or first with a salt and 

 then with an alkali, or with a basic oxide and water, &c.) the phlorine 

 forms a salt with its metal, and the hydroxyl radicle takes the place of 

 the chlorine for example, CH 3 -OH is obtained from CH 3 C1. By the 

 action of metallic derivatives of hydrocarbons for example, CH 3 Na 

 the chlorine 'also gives a salt, and the hydrocarbon radicle for instance, 



