Introduction. 15 



is a yellowish green gas with an irritating and suffocating smell, 

 very soluble in water and of great chemical activity. The prop- 

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

 bleaching, disinfecting and deodorizing powers. It readily de- 

 stroys most coloring matters and is largely employed in bleaching 

 vegetable textile fabrics, as cotton or linen. It cannot be used for 

 woolen or silk fabrics, as it injures the fibres themselves. Chlor- 

 ine only bleaches in the presence of water and it really acts by 

 decomposing the water, with formation of oxygen, which is the 

 active agent. Its action as a disinfectant is probably due to the 

 same process, the oxygen of the water combining with the or- 

 ganic matter and micro-organisms and destroying them. 



Chlorine is present in all soils, generally in combination with 

 sodium, as sodium chloride. It is present in all plants, although 

 its necessity for plant growth may be questioned. Crops have 

 been brought to maturity in its entire absence. Chlorine with 

 sodium, as common salt, is sometimes used as an indirect fertilzer. 



Silicon (Si) is extremely abundant in the rocks of the earth's 

 crust, and though it forms a very important ingredient in soils 

 and occurs in most plant ashes, it does not appear to be abso- 

 lutely essential as a plant food. Some recent work, however, has 

 shown that soluble silica in a soil enables a plant to subsist in the 

 presence of a smaller quantity of phosphoric acid than would be 

 necessary without the silica. 



The element itself is a brown solid and at one time was difficult 

 to prepare in any quantity. At present, with the electric fur- 

 nace, it is easily produced and its price per pound has been 

 greatly reduced. 



The oxide, called silica, is a compound of silicon and oxygen 

 and is a very abundant substance, occurring free as quartz, flint 

 and sand ; in combination with metals the very numerous and im- 

 portant substances called silicates, are produced. It has been 

 estimated that nearly half of the solid mass of the earth's crust 

 consists of silica. 



