DEFINITIONS AND DESCRIPTIONS. 137 



color. Those of lime, magnesia, etc. are of a yellow color, and 

 soluble in water ; but the subsalts are insoluble. 



5. ^pocrenate of alumina, when neutral, is insoluble ; but 

 soluble when there is an excess of acid. Jlpocrenate of the pro- 

 toxide of iron is soluble, but the salt of the peroxide is insoluble 

 in water; hence this acid with its various salts are generally 

 soluble in water to some extent, and must be conveyed into the 

 organs of plants. 



6. Extract of humus and glarin are brown matters composed 

 mostly of carbon, hydrogen and oxygen. The above substances 

 form the chief ingredients of vegetable vwuld, and pass into 

 each other in the changes which take place in the soil. 



When plants are subjected to ultimate analysis, that is, re- 

 solved into their simple constituents, they are found to be 

 composed of carbon, oxygen, hydrogen and nitrogen, which 

 form the vegetable proximate principles ; together with phos- 

 phorus, silicon and sulphur ; the alkalies, soda, potassa, ammo- 

 nia ; the alkaline earths, magnesia, lime and alumina, and the 

 oxides of iron, and of manganese. The four simple substances, 

 which form the proximate principles, are called the organic^ 

 and the remaining bodies the inorganic constituents of plants. 



The simple bodies of which vegetables are composed, are, 

 of course, all or nearly all derived from a source foreign to 

 the plant ; for although the vital power, may combine these 

 simple elements, so as to form a great variety of different 

 compounds, it cannot create a single particle of matter. The 

 view which was formerly taken, that metallic oxides were the 

 products of the vital power, has no foundation, either in fact, 

 or in philosophy. Whence then do plants derive the mate- 

 rials, out of which, their vital functions build up their vegeta- 

 ble structure ? In what particular form do they enter the 

 organs of plants, and what are the changes which take place 

 in their assimilation ? These questions have been variously 

 answered, and some things are still matters of controversy. It 

 will be necessary to devote this and the following sections to a 

 12 



