SOIL ITS ANALYSIS. 213 



presence of phosphoric acid may otherwise be determined by treat- 

 ing the phosphate of iron in solution in the way which I have already 

 indicated. 



From what precedes, it must be obvious that the most carefully 

 conducted chemical analysis of a soil, only leads us to the discovery 

 of certain principles which exist in very small quantity, although 

 their action is unquestionably useful to vegetation. As to the de- 

 termination 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 elementary analyses to determine 

 the precise composition of these substances. The finest part car- 

 ried off by the water will always show properties analogous to those 

 of clay ; the sand, which is generally silicious, will exhibit the char- 

 acters 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 connection with the mineral constitution of ara- 

 ble soils, to expose very briefly the general properties of clay or 

 loam, of quartz, and of carbonate of lime, substances in fact which 

 form the bases of all arable lands. Pure clay composed of silica, 

 alumina, and water, does not contain these substances in the state 

 of simple mixture. The inquiries of M. Berthier have satisfactorily 

 shown that clay is an 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 alkaline solution, which would not be the case were the silica 

 present in the state of quartzy sand, however fine. 



Pure clays are white, unctuous to the touch, stick to the tongue 

 when dry, and when breathed upon give out an odor which is well 

 known, and is commonly spoken of as the argillaceous odor. 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 directions. It is to an action 

 of this kind that we must ascribe the cracks and deep fissures which 

 traverse our clayey soils in all directions during the contiiniance of 

 great droughts. 



The constitutional water of clays is retained by a very powerful 

 affinity, and does not separate under a red heat ; pure clay has a 

 specific gravity of about 2.5 ; but the weight is frequently modified 

 by the presence of foreign matter, for it contains sand, metj'lic 

 oxides, carbonate of lime, carbonate of magnesia, and frequently 

 even combustible substances from bitumen to plumbago, all of which 

 admixtures of course modify the properties which are most highly 

 esteemed in clays, such as fineness, whiteness, infusibility, &c. 



Quartz is abundantly distributed throughout nature, and is met 

 with in very different states in the form of transparent colorless 

 crystals constituting rock crystals, as sand of different fineness ; 

 finally, in masses constituting true rocks. Quartz is the silica of 



