TIIK HALOdKNs 465 



-HI -boil, it decomposes hydrocarbons (and many of their derivatives) at 

 ^i high temperature, depriving them of their hydrogen and liberating 

 the earbon, as, for example, is clearly seen when a lighted candle is 

 placed in a vessel containing chlorine. The flame becomes smaller but 

 continues for a certain time, a large amount of soot is obtained, and 

 hydrochloric acid is formed. In this case the gaseous and incandescent 

 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, Arc., proceeds otherwise at lower tempera- 

 tures, as we will proceed to consider. 



A very important epoch in the history of chemistry was formed 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 Le 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, because 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 metalepsis. The mechanism of this substitution 

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

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

 on the one hand hydrochloric acid and 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 

 >f Itifilrochloric acid.' 22 The scheme of the process is as follows : 

 C M H IM X + C1 2 C^H^CIX + HC1 



Hydrocarbon. Free chlorine. Product of metalepsis. Hydrochloric acid. 



21 The same 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 oxygen compete 

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

 with the hydrogen ; if pure hydrogen be taken with a sufficient quantity of chlorine, it 

 will all combine with the chlorine without forming any water with the oxygen. 



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

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

 molecule of chlorine (p. 808) contains two atoms of this substance ; one atom replaces 

 tin- hydrogen, and the other combines with it. 



VOL. I. 



