22 ELEMENTARY AGRICULTURAL CHEMISTRY 



Chlorine is Tery abundant, especially in combination with 

 sodium, as rock-salt^ in the sea and in spring- watei ^ Other 

 compounds of chlorine also occur as minerals. The element 

 is usually obtained by oxidising hydrochloric acid, HCl, when 

 the hydrogen is removed to form water and the chlorine 

 evolved. Many substances may be used to bring about this 

 oxidation. Black oxide of manganese, MnO,, is often used. 

 When this substance is heated with a solution of hydrochloric 

 acid (the usual hydrochloric acid, or " spirits of salt ") about 

 half the chlorine present is evolved, and the gas, being nearly 

 two and a half times as heavy as air, can be collected by leading 

 it to the bottom of upright vessels. 



MnO, + 4HC1 = MnCl, + CI, + 2H,0. 



Chlorine is a yellowish-green gas, possessed of an irritating 

 and suffocating smell, very soluble in water, and of great 

 chemical activity. It readily unites with most metals, and 

 shows a particularly strong tendency to combine with hydrogen. 

 By pressure and cold, chlorine can be liquefied. The properties 

 of chlorine which are most valued in the arts are its bleaching, 

 disinfecting and deodorising powers. It readily destroys 

 most organic colouring matters, and is largely employed in 

 bleaching vegetable textile fabrics, e.g,^ cotton or linen, lb 

 cannot be used for woollen or silk fabrics, as it injures the 

 fibres themselves. Chlorine only bleaches in the presence cf 

 water, and it really acts by oxidation. Of itself it does not 

 decompose in water, except in the presence of strong light (sun- 

 light), but the combined effect of the organic colouring matter 

 tending to combine with oxygen and the attraction of chlorine 

 for hydrogen bring about the decomposition of water, with the 

 production of some oxidised organic matter, which is generally 

 colourless, and hydrochloric acid. Its action as a disinfectant 

 is probably due to the same process, the oxygen of water com- 

 bining with the organic matters, and micro-organisms and 

 destroying them. 



Silicon is extremely abundant in the rocks of the earth's 



