ANALYSIS. 



however, lead to error. To make 

 sure that tlie precipitate is formed by 

 a phospiialp, it must be rcdissolved in 

 hydrociilorio acid, and the acid solu- 

 tion evaporated to dryness, so as to 

 render the silica which may exist in 

 it insoluble. By treating the resi- 

 due with hydrochloric acid again, the 

 phosphates alone will be dissolved. 

 The presence of phosphoric acid may 

 otherwise be determined by treating 

 the phosphate of iron in solution in 

 the way which I have already indi- 

 cated. 



From what precedes, it must be ob- 

 vious that the most carefully conduct- 

 ed chemical analysis of a soil only 

 leads us to the discovery of certain 

 principles which exist in very small 

 quantity, although their action is un- 

 questionably useful to vegetation. As 

 to the detennination of the relative 

 quantities of sand and loam, this rests 

 upon simple washing ; and a chemist 

 would spend his time to very little 

 purpose in seeking, by means of ele- 

 mentary analyses, to determine the 

 precise composition of these substan- 

 ces. The finest part, carried off by 

 the water, will always show proper- 

 ties analogous to those of clay ; tlie 

 sand, which is generally silicious, will 

 exhibit the characters of quartz ; and 

 the calcareous fragments which are 

 mixed with it will exhibit those that 

 belong to carbonate of lime. It will 

 be sufficient, then, in connexion with 

 the mineral constitution of arable 

 soils, to expose very briefly the gen- 

 eral properties of clay or loam, of 

 quartz, and of carbonate of lime, sub- 

 stances, in fact, which form the bases 

 of all arable lands. Pure clay, com- 

 posed of silica, alumina, and water, 

 does not contam these substances in 

 the state of simple mixture. The in- 

 quiries of M. Berthier have satisfac- 

 torily shown that clay is a hydrated 

 silicate of alumina. "When we re- 

 move a portion of the alumina from 

 clay, for example, by treating it with 

 a strong acid, the silica which is set 

 at liberty will dissolve in an alkaUne 

 solution, which would not be the case 

 were the silica present in the state 

 of quartzy sand, however fiue. 

 20 



Pure clays are white, unctuous to 

 the touch, stick to the tongue when 

 dry, and when breathed upon, give 

 out an odour which is well known, 

 and is commonly spoken of as the 

 argillaceous odour. This property of 

 dry clay to adhere to the tongue is 

 owing to its avidity for water. It is 

 known, in fact, that dry clay, brought 

 into contact with water, first swells, 

 and finally mixes with it completely. 

 Duly moistened, it forms a tough and 

 eminently plastic mass. Exposed to 

 the air, moist clay, as it dries, shrinks 

 considerably ; and if the drying be 

 rapid, the mass cracks in all direc- 

 tions. It is to an action of this kind 

 that we must ascribe the cracks and 

 deep fissures which traverse our clay- 

 ey soils in all directions during the 

 continuance of great droughts. 



The constitutional water of clays 

 is retained by a very powerful affin- 

 ity, and does not separate under a red 

 heat ; pure clay has a specific gravity 

 of about 25 ; but the weight is fre- 

 quently modified by the presence of 

 foreign matter, for it contains sand, 

 metallic oxides, carbonate of lime, 

 carbonate of magnesia, and frequent- 

 ly even combustible substances, from 

 bitumen to plumbago, all of which ad- 

 mixtures of course modify the prop- 

 erties which are most highly esteem- 

 ed in clays, such as fineness, white- 

 ness, ini'usibility, &c. 



Quartz is abundantly distributed 

 throughout nature, and is met with in 

 very different states : in the form of 

 transparent colourless crystals, con- 

 stituting rock crystals, as sand of dif- 

 ferent fineness ; finally, in masses, 

 constituting true rocks. . Quartz is 

 the silica of chemists, and a com- 

 pound, according to them, of oxygen 

 and silicon, in the proportion, Berze- 

 lius says, of 100 of the radical to 108 

 of oxygen. 



Silica, in a state of purity, occurs 

 in the form of a white powder, and 

 having a density of 2-7. It is infusi- 

 ble in the most violent furnace ; but 

 it not only melts in the intense heat 

 which results from the combustion 

 of a mixture of hydrogen and oxygen 

 gas, but it is even dissipated in v,i- 



