AGRICULTURAL CHEMISTRY. 



45 



sufficiently obvious; a substance wbich 

 had been used with success in gardening, 

 must have soon been tried in farming; and 

 in countries where marie was not to be 

 found, calcined limestone would be natu- 

 rally employed as a substitute. 



The elder writers on agriculture had no 

 correct notions of the nature of lime, lime- 

 stone, and marie, or of their effects; and 

 this was the necessary consequence of the 

 imperfection of the chemistry of the age. 

 Calcarious matter was considered by the 

 alchymists as a peculiar earth, which in 

 the fire became combined with inflamma- 

 .ble acid; and Evelyn and Hartlib, and still 

 later, Lisle, in their works on husbandry, 

 have characterized it merely as a hot ma- 

 nure of use in cold lands. It is to Dr. 

 Black of Edinburg that our first distinct 

 rudiments of knowledge on the subject 

 are owing. 



About the year 1755, this celebrated 

 professor proved, by the most decisive ex- 

 periments, that limestone and all its mod- 

 ifications, marbles, chalks, and marles, 

 consist principally of a peculiar earth uni- 

 ted to an aerial acid: that the acid is given 

 out in burning, occasioning a loss of more 

 than 40 per cent, and that the lime in con- 

 sequence becomes caustic. 



These important facts immediately ap- 

 plied with equal certainty to the explana- 

 tion of the uses of lime, both as a cement 

 and as a manure. As a cement, lime ap- 

 plied in its caustic state acquires its hard- 

 ness and durability, by absorbing the ae- 

 rial (or as it has been since called, carbon- 

 ic) acid, which always exists in small 

 quantities in the atmosphere, it becomes 

 as it were again limestone. Chalks, cal- 

 careous marles, or powdered limestones, 

 act merely by forming an useful earthy 

 ingredient of the soil, and their efficacy 

 is proportioned to the deficiency of calca- 

 reous matter, which in larger or smaller 

 quantities seems to be an essential ingre- 

 dient of all fertile soils; necessary perhaps 

 to their proper texture, and as an ingredi- 

 ent in the organs of plants. Burnt lime, 

 in its first efiect, acts as a decomposing 

 agent upon animal or vegetable matter, 

 and seems to bring it into a state on which 

 it becomes more rapidly a vegetable nour- 

 ishment; gradually, however, the lime is 

 neutralized by carbonic acid, and convert- 

 ed into a substance analagous to chalk; but 



in this case it more perfectly mixes with 

 the other ingredients of the soil, is more 

 generally difl'used and finely divided, and 

 it is probably more useful to land than any 

 calcareous substance in its natural state. 

 The most considerable fact made known 

 with regard to limestone within the last 

 few years, is owing to Mr. Tennant. It 

 had been long known that a particular 

 species of limestone found in different parls^ 

 of the North of England, when applied in 

 its burnt and slaked state to land in con- 

 siderable quantities, occasioned sterility, 

 or considerably injured the crops for ma- 

 ny years. Mr. Tennant in 1800, by a 

 chemical examination of this species of 

 limestone, ascertained, that it differed from' 

 common limestones by containing magne- 

 sian earth; and by several experiments he 

 proved that this earth was prejudicial to 

 vegetation, when applied in large quanti- 

 ties in its caustic state. Under common 

 circumstances, the lime from the magne- 

 sian limestone is,- however, used in mod- 

 erate quantities upon fertile soils in Lei- 

 cestershire, Derbyshire, and Yorkshire, 

 with good effect; and it may be applied in 

 greater quantities to soils containing very 

 large proportions of vegetable matter. 

 Magnesia, when combined with carbonic 

 acid gas, seems not to be prejudicial to 

 vegetation, and in soils rich in manure, it 

 is speedily supplied with this principle 

 from the decomposition of the manure. 

 After the nature and operation of manures, 

 have been discussed, the next, and last 

 subject for our consideration, will be some 

 of the operations of husbandry capable oF 

 elucidation by chemical principles. The 

 chemical theory of fallowing is very 

 simple. Fallowing affords no new source" 

 of riches to the soil. It merely tends ta 

 produce an accum.ulation of decomposing 

 matter, which in the common course of 

 crops, would be employed as it is formed, 

 and it is scarcely possible to imagine a 

 single instance of a cultivated soil, which 

 can be supposed to remain fallow for a 

 year with advantage to the farmer. The 

 only cases where this practice is beneficial 

 seems to be in the destruction of weeds, 

 and for cleansing foul soils. The chem- 

 ical theory of paring and burning, I shall 

 fully discuss in this part of the course. 



It is obvious that in all cases it must 

 destroy a certain quantity of vegetable 



