SODIUM, PHOSPHATES OF SODA, AND CARBONATE OF LIME. 193 



ly supply 3oda to the growing plant, to which soda is necessary, but it 

 also acts upon certain other substances which the plants require, so as 

 to render them soluble, and to facilitate their entrance into the roots of 

 plants. To the [)resence of soda in this caustic state, the efficacy of 

 such composts of common salt and lime in promoting vegetation, is in 

 part to be ascribed. 



6°. Sodium is a soft metal of a silver white colour, and, like potassi- 

 um, light enough to float upon water. It is obtained by heating caustic 

 soda with a mixture of charcoal and iron filings. It takes fire upon 

 water — though not so readily as potassium — and combines with its oxy- 

 gen to form soda. In the metallic state it is not known to occur in na- 

 ture, and, therefore, does not directly act upon vegetation. With oxy- 

 gen it forms soda, — with chlorine, chloride of sodium (common salt), — 

 and with sulptiur, sulphuret of sodium, — all of which, as already stated^ 

 are more or less beneficial to vegetation. 



7°. Phosphates of Soda. — When the common soda of the shops is added 

 to a solution of phosphoric acid in water, till effervescence ceases, and 

 the solution is evaporated to dryness, phos[)hate of soda is formed, and 

 by the subsequent addition of as much more phosphoric acid — 6i-phos- 

 phate. These salts occur more or less abundantly in the ash of nearly 

 all plants ; they are occasionally also detected in the soil, and one or 

 other of them is almost always present in urine and other animal ma- 

 nures. As we know from theory that these compounds must be grate- 

 ful to plants, we are justified in ascribing a portion of the efficacy of animal 

 manures, in promoting the growth of vegetables, to the presence of these 

 phosphates, as well as to that of the phosphates of potash (p. 190). 

 They are not known to occur in the mineral kingdom in any large quan- 

 tity, neither are they articles of manufacture, hence their direct action 

 upon vegetation has not hitherto been made the subject of separate ex- 

 periment. 



VII. CALCIUM, LIME, CARBONATE OF LIME, SULPHATE OF LIME, NI- 

 TRATE OF LlME, PHOSPHATES OF LIME, CHLORIDE OF CALCIUM, SUL- 

 PHURET OF CALCIUM. 



1°. Carbonate of Lime. — Chalk, marble, and nearly all the lime 

 stones in common use, are varieties, more or less pure, of that com 

 pound of lime with carbonic acid which is known to chemists as car- 

 bonate of lime. It occurs of various colours and of various degrees of 

 hardness, but in weight the compact varieties are very much alike, be- 

 ing generally a little more than 2i times (2*7) heavier than water. 

 They all dissolve with effervescence in dilute muriatic acid (spirit of 

 salt), and by the bubbles of gas which are seen to escape when a drop 

 of this acid is applied to them, limestones may in general be readily dis- 

 tinguished from other varieties of rock. They dissolve slowly also in 

 water which holds carbonic acid in solution ; and hence the springs 

 which issue from the neighbourhood of deposits of limestone are gene- 

 rally charged in a high degree with this mineral substance. 



The value of this carbonate of lime in rendering a soil capable of pro- 

 ducing and sustaining a luxuriant vegetation depends, in part, it is true, 

 on the necessity of a certain proportion of lime to the growth and full 

 developement of the several parts of nearly all plants, but it performs also 

 



