THE HALOGENS 471 



continues to burn- for a certain time, a large amount of soot is obtained, 

 and hydrochloric acid is formed. In this case the gaseous and incan- 

 descent substances of the flame are decomposed by the chlorine, the 

 hydrogen combines with it, and the carbon is disengaged as soot. 21 

 This action of chlorine on hydrocarbons, &c., proceeds otherwise at 

 lower temperatures, as we will now consider. 



A very important epoch in the history of chemistry was inaugurated 

 by the discovery of Dumas and Laurent that chlorine is able to displace 

 and replace hydrogen. This discovery is important from the fact that 

 chlorine proved to be an element which combines with great ease 

 simultaneously with both the hydrogen and the element with which 

 the hydrogen was combined. This clearly proved that there is no 

 opposite polarity between elements forming stable compounds. Chlorine 

 does not combine with hydrogen because it has opposite properties, as 

 Dumas and. Laurent stated previously, accounting hydrogen to be 

 electro-positive and chlorine electro-negative ; this is not the reason of 

 their combining together, for the same chlorine which combines with 

 hydrogen is also able to replace it without altering many of the 

 properties of the resultant substance. This substitution of hydrogen 

 "by chlorine is termed metalepnis. The mechanism of this substitution 

 is very constant. If we take a hydrogen compound, preferably a 

 hydrocarbon, and if chlorine acts directly on it, then there is produced 

 on the one hand hydrochloric acid an*d on the other hand a compound 

 .containing chlorine in the place of the hydrogen so that the chlorine 

 divides itself into two equal portions, one portion is evolved as hydro- 

 chloric acid, and the other portion takes the place of the hydrogen 

 thus liberated. Hence this metalepsis is always accompanied by the 

 formation of hydrochloric acid.* 2 The scheme of the process is as 

 follows : 



C n H m X + C1 2 = C n H MM ClX 4- HC1 



Hydrocarbon Free chlorine Product of metalepsis Hydrochloric acid 



Or, in general terms 



RH 4= C1 2 , t= RC1 + HC1. 



The conditions under which metalepsis takes place are also very 

 constant. In the dark chlorine does not usually act on hydrogen com- 



81 The same reaction takes place under the action of oxygen, with the difference that 

 it 'burns the carbon, which chlorine is not able to do. If chlorine and ox j gen compete 

 together at a high temperature, the oxygen will unite with the carbon, and the chlorine 

 Vrith the hydrogen. 



82 This division of chlorine into two portions may at the same time be taken as a clear 

 confirmation of the conception of molecules. According to Avogadro-Gerhardt's law, the 

 molecule of chlorine (p. 310) contains two atoms of this substance ; one atom replaces 



hydrogen, and the other combines with it. 



