- 21G GENERAL REMARKS ON THESE SILICATES. 



sail tself, when the ingredients of which tliey severally consist are na- 

 turally present in, or are artificially added to, the soil. Hence, the ad- 

 dition of potash or soda to the land may cause the production of sili- 

 cates of these alkalies — probahly soluble silicates — which water will 

 be capable of dissolving and bearing to the extremities of the roots. 

 Hence also, in a sandy soil, the addition of lime may give rise to the 

 production of insoluble silicates of this earth, — and the beneficial effect 

 of the lime upon the land may thus sooner cease to be observable than 

 in soils of a different character, where it is not so liable to be locked up 

 in an insoluble state of combination ; and 



2°. That with the exception of those of potash and soda, which con- 

 fain much alkali, these silicates are all insoluble in water, and thus not 

 directly available to the nutrition of plants. Except those of alumina, 

 however, they are all slowly decomposed by atmospheric agents, and 

 their constituent elements thus brought, to a certain extent, within the 

 reach of plants; while, without exception, they are all capable of de- 

 composition in the soil by the agency of the acid substances, chiefly or- 

 ganic, which there exisf, or which are produced during the growth and 

 decay of vegetable substances. From this latter source, the chief supply 

 of the ingredients contained in the silicates, is, in most soils, derived by 

 living plants. 



To this cause is attributed the surprising effect often observed to fol- 

 low from the addition of vegetable matter to a sandy soil on which a 

 prcN'ious addition of lime had ceased to produce any further beneficial 

 effect. The organic acids formed by the vegetable matter during its de- 

 cay desompose the silicates of lime previously produced, and thus liber- 

 ale the lime from its insoluble stale of combination. But when the sili- 

 cates have been all decomposed by this agency, the further addition of ve- 

 getable matter ceases necessarily to produce the same remarkable effects. 



XI. THE OXIDES, SULPHURETS, SULPHATES, AND CARBONATES OF IRON. 



1°. Oxides of Iron. — It is well known that when metallic iron is ex- 

 posed to moist air, it gradually rusts and becomes covered with, or whol- 

 ly changed into, a crumbling ochrey mass of a reddish brown colour. 

 This powder is a compound of iron and oxygen only, containing 69j per 

 cent, of the former, and 30| per cent, of the latter. 



When iron is heated in the smith's forge, and then beat on the anvil, a 

 scale flies off" which is of a black colour, and when crushed gives a black 

 powder. This also consists of iron and oxygen only, but the proportion 

 of oxygen is not so great as in the red powder above described. In^both 

 cases the iron has derived its oxygen from the atmosphere. 



To these compounds of iron, with oxygen, the name n£ oxides is given. 

 There are only two which are of interest to the agriculturist, namely, 



CONSISTING OP 

 / —  -/ ^ 



Iron, Oxygen. Symbol. Colour. 

 The ^rs« oxide* . . 77-23 22-77 Fe Of Black 

 The second oxide . 69-34 30-66 FeaOg Red. 



• The first is also called the prot-oxide, the secona either the sesqui, or more usuallt/ th9 

 oer oxide of iron, 

 t Iron is represented by the symbol Fe, the initial loUers of its Latin name (ferrum). 



