VII.] COMPOSITION OF PURE WATER. 109 



and, in order to complete the demonstration of the chemical 

 decomposition of water, it is necessary to explain how the 

 oxygen may be liberated. To obtain the oxygen in a free 

 fomi, it is clearly necessary to present to the water some 

 substance which has a strong attraction for hydrogen. Such 

 a substance is found in the gaseous element known to 

 chemists as chlorine. This body exists largely in common 

 salt, and in the well known substances " spirit of salt " and 

 " chloride of lime." When set free it is an extremely 

 poisonous gas, differing from all the gases to which we 

 have hitherto referred, in that it possesses a very marked 

 greenish-yellow colour, whence its name {yXwfioQ, chloros, 

 green). 



One of the most characteristic properties of chlorine is 

 its powerful attraction for hydrogen. Mix the two gases 

 together, and they combine with explosive violence, if 

 exposed to sunshine ; and, even in diffused daylight, they 

 slowly and quietly unite. This attraction for hydrogen 

 stands us in good stead when we wish to get oxygen from 

 water. A mixture of chlorine and steam is passed through 

 a strongly-heated tube, and the chlorine eagerly seizes on 

 the hydrogen to form a compound known as hydrochloric- 

 acid gas, while the oxygen is set free. A similar action is 

 indeed constantly being effected, in a less striking manner, 

 in many of the industrial arts. Chlorine is largely used as 

 a bleaching agent, but, in a dry state, it is powerless to 

 bleach ; it is only when it is associated with moisture that 

 it becomes active. When moist, however, it slowly 

 decomposes the water, combining with its hydrogen and 

 disengaging its oxygen ; and it is this oxygen, at the 

 moment of its liberation, which is the really active agent in 

 bleaching. 



The proof of the composition of water, derived from the 



