50 



AftRICULTURAL CHEMISTRY. 



From Sir Humphrey DavrsEieme.u8 of Agricultural I gredient^^ The earthy matters are the 



true basis of the soil; the other parts, 



Ciiemistr)' 

 AGRICULTURAL CHEMISTRY 



Continued from p rge 3. 



The vahie and uses of every species of 

 agricultural produce are most correctly 

 estimated and applied when practical 

 knowledge is assisted by principles deri- 

 ved from chemistry. The compounds in 

 vegetables, really nutritive as the food of 

 animals, are very few; farina or the pure 

 matter of starch, gluten, vegetable jelly, 

 and extract. Of these the most nutri 

 live is gluten, wh,ich approaches, nearest 

 in its nature to animal matter, and which 

 is the substance that gives to wheat its su- 

 periority over other grain. 



The next in order as to nourishinj 

 power is sugar, then farina; and lastot all 

 gelatinous and extractive matters. 



Simple tests of the relative nourishing 

 powers of the different species of food, 

 are the relative quantities of these sub- 

 stances that they afford by analysis; and 

 though taste and appearance must in- 

 fluence the consumption of all articles in 

 years of plenty, yet they are less attended 

 to in times of scarcity, and on such occa- 

 sions this kind of knowledge may be of 

 the greatest importance. 



Sugar and farina, or starch, are very 

 similar in composition, and are capable 

 of being converted into each other by 

 simple chemical processes. In the discus- 

 sion of their relationsj I shall detail to you 

 the results of some recent experiments 

 which will be found possessed of applica- 

 tions both- to the economy of vegetation, 

 and to some important processes of manu- 

 facture. 



All the varieties of substances found in 

 plants, are produced from the sap, and the 

 sap of plants is derived from water or from 

 the fluids in the soil, and it is altered by. 

 or combined with principles derived from 

 the atmosphere. The influence of the 

 soil, of water, and of air, will therefore 

 be the next subject of consideration. Soils 

 in all cases consist of a mixture of differ- 

 ent finely divided earthy matters; with 

 animal or vegetable substances in a state 

 of decomposition, and certain saline in- 



* Note — In lines 2.3 & 24, p. 2, col. 2d, instead of 

 six and three substances, read seven are inflammable 

 bodies and two are gasses. 



whether natural or artificially introduced, 

 operate in the same manner as manures. 

 Four earths generally abound in soils, the 

 aluminous, the sileceous, the calcareous, 

 and the magnesian. These earths, as I 

 have discovered, consist of highly inflam- 

 mable metals united to a pure air or oxy- 

 gen, and they are not as far as we know, 

 decomposed or altered in vegetation. 



The great use of the soil is to afford 

 support to the plant, to enable it to fix its 

 roots, and to derive nourishment by its 

 tubes slowly and gradually, from the sol- 

 uble substances mixed with the earths. 



That a particular mixture of earths is 

 connected with fertility, cannot be doubt- 

 ed : and almost all sterile soils are capable 

 of being improved by a modification of 

 their earthy constituent parts. I shall de- 

 scribe the simplest methods as yet discov- 

 ered of analyzing soils, and of ascertaining 

 the constitution and chemical ingredients 

 which appear to be connected with fertil- 

 ity, and on this subject many of the for- 

 mer difficulties of investigation will be 

 found to be removed by recent enqui- 

 ries. 



The necessity of water to vegetation, 

 and the luxuriancy of the growth of 

 plants connected with the presence of 

 moisture in the southern countries of the 

 old continent, led to the opinion so prev- 

 alent in the early schools of philosophy, 

 that water was the great productive ele- 

 ment, the substance from which all things 

 were capable of being composed and in- 

 to which they finally resolved, " water is 

 the noblest," seems to have been an ex- 

 pression of this opinion, adopted by the 

 Greeks from the Egyptians taught by 

 Thales, and revived by the alchemists in 

 late times. Van Helmont in 1610, con- 

 ceived that he had proved by a decisive 

 experiment that all the products of veg- 

 etables were capable of being generated 

 from water. His results were shown to 

 be fallacious by Woodward in 1691, but 

 the true use of water in vegetation was 

 unknown till 1785, when Cavendish made 

 the grand discover}', that it was composed 

 of two elastic fluids or gasses, inflammable 

 gas or hydrogen, and vital gas or oxy- 

 gen. 



Air, like water, was regarded as a pure 



