BOOK II. SPECIES OF MINERAL MANURES. 247 



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

 portions 34. 



1 180. 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 gyp- 

 sum 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 property 

 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. In America it is employed with signal success ; it has been 

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

 in various ways, and upon different crops. 



1181. Very discordant notions have been formed as to the mode of operation 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 at- 

 traction 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. 



1182. The ashes of saintfoin, clover, and rye-grass, afford considerable quantities ofgyjisum ; 

 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 esti- 

 mation, 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 cultivation, 

 gypsum is furnished 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 con- 

 firmed 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 Oxford- 

 shire, Glocestershire, Somersetshire, Derbyshire, Yorkshire, &c. and requires only pul- 

 verisation for its preparation. 



1 183. Upon 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 pro- 

 ducing 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 con- 

 sists 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. 



1 184. Vitriolic impregnations in soils where there is no calcareous matter are injurious ; 

 but it is probably in consequence of their supplying an excess of ferruginous matter to 

 the sap. Oxide of iron, in small quantities, forms a useful part of soils ; it is found 

 in the ashes of plants, and probably is hurtful only in its acid combinations. The ashes 

 of all peats do not afford gypsum. In general, when a recent peat-ash emits a strong 

 smell, resembling that of rotten eggs, when acted upon by vinegar, it will furnish 

 gypsum. 



1 185. Phosphate of lime is a combination of phosphoric acid and lime, one proportion of 

 each. It is a compound insoluble in pure water, but soluble in water containing any acid 

 matter. It forms the greatest part of calcined bones. It exists in most excrementitious 

 substances, and is found both in the straw and grain of wheat, barley, oats, and rye, and 

 likewise in beans, peas, and tares. It exists in some places in these islands native, but 

 only in very small quantities. Phosphate of lime is generally conveyed to the land in 

 the composition of other manure, and it is probably necessary to corn crops and other 

 white crops. 



1186. JBone-ashes calcined and ground to powder will probably be found useful on arable 

 lands containing much vegetable matter, and may perhaps enable soft peats to produce 



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