SILICON TITANIUM CHLORINE FLUORINE. 17 



Aluminium is apparently not a plant food, though the ashes 

 of some few plants contain it in small quantities. 



Silicon always occurs in combination, either with oxygen as 

 silica, SiO. 2 , which is found as quartz, flint, sand, c., or with 

 oxygen and metals as the very numerous and abundant sili- 

 cates, e.g., felspar, mica, c., &c. 



Sand, which consists of little fragments of quartz, is very 

 permanent and is little affected by water or carbon dioxide. 



Sand forms the largest portion of most soils and, if pure 

 quartz, is devoid of plant food. It greatly affects the porosity 

 and general texture of the soil. The silica which many plants 

 contain is not believed to be essential to their growth; it 

 probably is taken in by the roots of the plant in the form of 

 soluble silicates or the soluble silica formed, not from the sand 

 itself, but by the decomposition of silicates. 



Titanium, which resembles silicon in its chemical functions, 

 is not nearly so abundant. Its presence in the plant is usually 

 overlooked, though according to Wait* it is almost invariably 

 present in plant ash. 



Chlorine is an element possessed of remarkable and well- 

 known properties. It rarely occurs in the free state in nature, 

 but in the form of metallic chlorides is very abundant. This 

 is particularly true of sodium chloride, Nad, which is found in 

 sea and most spring water and as rock-salt. Chlorine is an 

 essential constituent of plants, and in some crops, e.g., mangels, 

 it occurs in large quantity. 



Fluorine occurs mainly as calcium fluoride, CaF 2 , in the 

 mineral known as fluor-spar. It is also present in almost all 

 naturally occurring forms of calcium phosphate, and doubtless, 

 in quantities too small to be readily detected, in many other 

 minerals. The element itself is difficult to prepare owing to 

 its great chemical activity. 



It is found in combination in the bones, blood, and urine of 



Jour. Amer. Chem. Soc., 1896, 18, 402. 



