4 PRELIMINARY OBSERVATIONS. 



It u a remarkable fact that those elements which are of the highest importance in the 

 animal and vegetable economy, form but an inconsiderable part of the soil. The tissues 

 of the animal frame are composed of the salts of phosphorus, which, in order to be detected 

 in the earth, require the utmost nicety in chemical manipulation ; while the compounds of 

 lime, magnesia and silica, are fur more abundant. 



It can not be said that the elements forming the essential part of the soil are numerous ; 

 and hence it may be inferred, that the elements which constitute the frame work of vege- 

 tables and animals are also comparatively few in number. Of those which exist in the 

 soil, the following enumeration comprehends the entire list, so far at least as can be re- 

 garded as essential to organization : Silcx, alumina, lime, magnesia, iron, manganese, 

 potash, soda, phosphoric acid, sulphur, chlorine, carbonic acid, ammonia, the elements of 

 water and air, and nitrogen, oxygen, hydrogen and carbon in an oxidized state. They may 

 be regarded as forming very naturally two classes of bodies : those which form the frame 

 work of the tissues ; and those which fill up, as it were, the interstices. In the former, or in 

 the frame work of the tissues, we find silex, lime, magnesia in combination with phosphoric 

 and carbonic acids ; in the latter, carbon, hydrogen and nitrogen in various combinations 

 are found. It is an important fact, established by the analyses which I am about to give, 

 that each tissue has its own distinct organization ; that they respectively possess their ele- 

 mentary bodies in proportions peculiar to themselves. For example, the constituents of 

 the leaf of a currant differ from those of the stem, bark, flower or wood ; the kernel of 

 wheat, from its envelopes, the husk or stalk. But what is practically still more interesting 

 and important, these component parts may be modified by culture and by soil. The law, 

 however, that certain elements or bodies are determined towards specific parts, is un- 

 affected : that the phosphates of magnesia and lime are determined to the essentially 

 nutritive parts of the plants, is a universal fact ; tiie modifications produced by a rich soil 

 and high culture are mainly seen in their accumulated quantity, and it is not very unlike 

 that process called the laying on of fat in certain parts by some breeds of domestic ani- 

 mals. We shall have occasion to treat of this interesting subject more at large, being 

 now more immediately concerned with the elementary bodies alluded to in the preceding 

 paragraphs. 



SILEX, SILICA, SILICIC ACID. 



These three names are synonymous. The substance is familiar to all, in the forms and 

 conditions of white sand, flint, and crystals of quartz. In those forms it is conmionly re- 

 garded as pure silex, and is so called ; but when it is combined with other bodies, or is in 

 a soluble state, it is called silicic acid, because it performs the functions of that class of 

 bodies which are known as acids, and not because it has the taste which acids are com- 

 monly supposed to possess ; for it is really tasteless, and produces no change upon litmus. 



Silex ii scarcely soluble in water, or in the ordinary acids ; but it is perfectly soluble in 

 fluoric acid, and in potash when aided by heat. The alkalies give it a solubility which it 



