240 DR. DAUBENY ON THE ROTATION OF CROPS, ETC. 



The latter, obtained from potatoes, yielded about 3*43 parts of inorganic matter in 

 the 1000, of which only 4*77 per cent, was phosphoric acid, whilst 82*48 was car- 

 bonate of lime. 



Thus, in the case of wheat, any condition of things which should check the forma- 

 tion of gluten, would diminish the quantity of phosphoric acid present in the ashes 

 of this plant, even thougli the soil might contain an abundant supply of that ingre- 

 dient; and as tlie formation of gluten is promoted by the presence of manures abound- 

 ing in ammoniacal salts*, so it may easily happen, that this principle should be 

 deficient where such manures are too sparingly administered. 



In a similar way, a variation in the constituents of barley and other crops may 

 be supposed to arise, not only from a larger or smaller supply of inorganic principles 

 in the soil, in the manner that Liebig has so lucidly explained to us, but likewise 

 from a more plentiful exhibition of those products of the decomposition of organic 

 bodies, which favour the development of particular organs, or of certain of the prox- 

 imate principles which the latter contain. 



Which, however, of tliese two suppositions applies to the cases now under con- 

 sideration, will be better seen, wlien we have considered the composition of the soil 

 in which they grew, as determined by analysis. 



PART III. 



On the chemical composition of the soil in tvhich the crops were grown, and on the 

 proportion of its ingredients that were available for the purposes of vegetation. 



The chief difficulty, which occurs with respect to the analysis of a soil, relates to the 

 determination of those ingredients which, like the phosphates and the alkalies, exist 

 in minute proportions, and which accordingly appear to have been overlooked by 

 Davy, and others, who first applied themselves to the subject of agricultural chemistry. 



It will not be necessary therefore to take up the time of this Society, by giving a 

 detailed account of the method pursued by Mr. Way in his examination of the soils 

 of which I wished to learn the composition ; it may be sufficient to state, that after 

 separating the several portions, one from the other, by the mechanical method pointed 

 out by Mr. Rham, and determining the relation which the coarser bore to the finer, 

 the latter, which alone were supposed capable of imparting any nourishment to plants, 

 at least within a limited period, was submitted to the usual course of examination 

 pursued by chemists. 



To ascertain the phosphates however, a distinct and a much larger portion of the soil 

 was operated upon, not less than 2000 grains being taken for the purpose, and this 

 was digested for five hours in water acidulated with muriatic acid, the flask employed 

 for the purpose being fitted up with a funnel attached to its neck, in the manner re- 



* See Hermbstadt's experiments quoted in the third of my Lectures on Agriculture, and Sir H. Davt's 

 Lectures on Agricultural Chemistry. 



