340 SCIENCE OF AGRICULTlTRE. Part II. 



Durham, and Yorkshire ; and in many parts of Ireland, particularly near Belfast. In 

 general, when limestones are not magnesian, their purity will be indicated by their loss 

 of weight in burning ; the more they lose, the larger is the quantity of calcareous 

 matter they contain. The magnesian limestones contain more carbonic acid than the 

 common limestones ; and I have found all of them lose more than half their weight by 

 calcination. 



2229. Gypsum. Besides being used in the forms of lime and carbonate of lime, cal- 

 careous matter is applied for the purposes of agriculture in other combinations. One of 

 these bodies is gypsum or sulphate of lime. This substance consists of sulphuric acid 

 (the same body that exists combined with water in oil of vitriol, and lime ; and when dry 

 it is composed of 55 parts of lime and 75 parts of sulphuric acid. Common gypsum or 

 selenite, such as that found at Shotover Hill, near Oxford, contains, besides sulphuric 

 acid and lime, a considerable quantity of water; and its composition may be thus 

 expressed : sulphuric acid one proportion 75 ; lime one proportion 55 ; water two pro- 

 portions 34. 



2230. The nature of gypsum is easily demonstrated ; if oil of vitriol be added to 

 quick-lime, there is a violent heat produced ; when the mixture is ignited, water is given 

 off, and gypsum alone is the result, if the acid has been used in sufficient quantity ; and 

 gypsum mixed with quick -lime, if the quantity has been deficient. Gypsum, free from 

 water, is sometimes found in nature, when it is called anhydrous selenite. It is distin- 

 guished from common gypsum by giving off no water when heated. When gypsum, free 

 from water, or deprived of water by heat, is made into a paste with water, it rapidly sets 

 by combining with that fluid. Plaster of Paris is powdered dry gypsum, and its pro- 

 perty as a cement, and its use in making casts, depends upon its solidifying a certain 

 quantity of water, and making with it a coherent mass. Gypsum is soluble in about 500 

 times its weight of cold water, and is more soluble in hot water ; so that when water has 

 been boiled in contact with gypsum, crystals of this substance are deposited as the water 

 cools. Gypsum is easily distinguished by its properties of affording precipitates to 

 solutions of oxalates and of barytic salts. It has been much used in America, where 

 it was first introduced by Franklin on his return from Paris, who had been much 

 struck with its effects there. He sowed the words. This has been sown with gypsum, on 

 a field of lucern, near Washington ; the effects astonished every passenger, and the use 

 of the manure quickly became general, and signally efficacious. It has been advan- 

 tageously used in Kent, but in most counties of England it has failed, though tried in 

 various ways, and upon different crops. 



2231. Very discordant notions have been formed as to the mode of ojieration of gypsum. 

 It has been supposed by some persons to act by its power of attracting moisture from the 

 air ; but this agency must be comparatively insignificant. When combined with water, 

 it retains that fluid too powerfully to yield it to the roots of the plant, and its adhesive 

 attraction for moisture is inconsiderable ; the small quantity in which it is used likewise 

 is a circumstance hostile to this idea. It has been erroneously said that gypsum assists 

 the putrefaction of animal substances, and the decomposition of manure. 



2232. The ashes of saintfoin, clover, and rye-grass, afford considerable quantities of 

 gypsum ; and the substance probably is intimately combined as a necessary part of their 

 woody fibre. If this be allowed, it is easy to explain the reason why it operates in such 

 small quantities ; for the whole of a clover crop, or saintfoin crop, on an acre, according 

 to estimation, would afford by incineration only three or four bushels of gypsum. The 

 reason why gypsum is not generally efficacious, is probably because most cultivated soils 

 contain it in sufficient quantities for the use of the grasses. In the common course of cul- 

 tivation, gypsum is furr ished in the manure ; for it is contained in stable dung, and in 

 the dung of all cattle fed on grass ; and it is not taken up in corn crops, or crops of peas 

 and beans, and in very small quantities in turnip crops ; but where lands are exclusively 

 devoted to pasturage and hay, it will be continually consumed. Should these statements 

 be confirmed by future enquiries, a practical inference of some value may be derived from 

 them. It is possible that lands which have ceased to bear good crops of clover, or artificial 

 grasses, may be restored by being manured with gypsum. This substance is found in 

 Oxfordshire, Gloucestershire, Somersetshire, Derbyshire, Yorkshire, &c. and requires 

 only pulverisation for its preparation. 



2233. Uimn the use of sulphate of iron, or green vitriol, which is a salt produced from 

 peat in Bedfordshire, some very interesting documents have been produced by Dr. 

 Pearson ; and there is little doubt that the peat salt and the vitriolic water acted chiefly 

 by producing gypsum. The soils on which both are efficacious are calcareous ; and 

 sulphate of iron is decomposed by the carbonate of lime in such soils. The sulphate of 

 iron consists of sulphuric acid and oxide of iron, and is an acid and a very soluble salt ; 

 when a solution of it is mixed with carbonate of lime, the sulphuric acid quits the oxide 

 of iron to unite to the lime, and the compounds produced are insipid and comparatively 

 insoluble. 



