348 PHOSPHATE OF LIME AND EARTH OF BOKES. 



chlorides also — and of giving ofT their chlorine into the surrounding air. 

 When they have been introduced into the sap therefore, by the roots, the 

 plant first appropsiates so much of the chlorine they contain as is neces- 

 sary for the supply of its natural wants, and evolves the rest. When 

 common salt is thus decomposed, soda remains behind in the sap, and 

 this is either worked up into the substance of the plant, or performs one 

 or other of those indirect functions 1 have already explained to you 

 (p. 328) when illustrating the probable action of potash and soda upon 

 the vegetable economy. When the other chlorides (of calcium or mag- 

 nesium) are decomposed, lime or magnesia remains in the sap, and is 

 in like manner either used up directly in the formation of the young 

 stem and seed, or is employed indirectly in promoting the chemical 

 changes that are continually going on in the sap. The living plant, 

 when in a healthy state, is probably endowed with the power of admit- 

 ting into its circulation, and of then decomposing and retaining, so much 

 only of these several chlorides, or of their constituents, as is fitted to 

 'enable its several organs to perform their functions in the most perfect 

 manner. 



In the soil itself, in the presence of organic matter of animal and 

 vegetable origin, common salt is fitted to promote certain chemical 

 changes, such as the production of alkaline nitrates — and probably sili- 

 cates — by which the growth of various kinds of plants is in a greater 

 or less degree increased. In the soil, also, from thek tendency to deli- 

 quesce, or run into a liquid, all these chlorides attract water from the 

 air, and thus help to keep the soil in a moister state. When applied in 

 sufficient quantity they destroy both animal and vegetable life, and 

 have, in consequence, been often used with advantage for the extirpa- 

 tion of weeds, and for the destruction of grubs and other vermin that 

 infest the land. 



9°. Phosphate of Lime and Earth of Bones. — The cattle that graze 

 in our fields derive, as you know, all the earthy materials of which cer- 

 tain parts of their bodies consist from the vegetables on which they feed. 

 These vegetables again must derive them from the soil. Thus the 

 earth of bones, or the phosphoric acid and lime of which it consists 

 (p. 196), must exist in the soil on which nutritive plants grow, and it 

 must occasionally occur that a soil will be deficient in these substances, 

 and will, therefore, supply them with difficulty lo the crops it rears. 

 The benefit which in this country is so often experienced from the use 

 of bones as a manure, has been ascribed, in part, to the supply of bone- 

 earth, with which it enriches the land. (See Appendix, No. I.) It 

 is not, however, to be inferred from this, that wherever bones are use- 

 ful, the application of bone-earth alone — in the form of burned bones, 

 or of the native phosphate of lime, (p. 199,) will necessarily prove 

 advantageous also. Burned bones were formerly employed in Eng- 

 land, but the practice has gradually fallen into disuse, and the same is, 

 I believe, the case in Germany. This is no proof, however, that the 

 native phosphate of Estremadura — already, it is said, imported into 

 Ireland for agricultural purposes, — would not benefit many soils if ap- 

 plied in the state of a sufficiently fine powder. Until carefully con- 

 ducted experunents, however, shall have been made, and the numerical 



