SULPHATE AND NITRATE OF LIME. 195 



crucioie It is sometimes produced in nature, where moist decaying 

 vegetable and animal matters are allowed to ferment in the presence of 

 gypsum ; it may sometimes also be delected in the soil, and in the waters 

 of mineral springs, and is contained largely in the recent refuse heaps 

 of tlie alkali works. Like the sulphurets of potassium and sodium, al- 

 ready described, it is fitted, when judiciously applied, to promote the 

 growth especially of those plants in which sulphur has been recognized 

 as a necessary constituent. 



6°. Sulphate of Lime, or gypsum, is a well known white crystalline 

 or earthy compound, which occurs as an abundant mineral deposit in 

 numerous parts of the globe. It is present in many soils, is contained 

 in the waters which percolate through such soils, and in those of springs 

 which ascend from rocky beds in which gypsum exists, and is detect- 

 ed in sensible proportions in the ashes of many cultivated plants. It 

 is extensively employed in the arts, and in some countries not less ex- 

 tensively as a means of promoting the fertility of the land. — [See Appen- 

 dix, p. 1.] 



The gypsum of commerce contains nearly 21 per cent, of its weight 

 of water, which it loses entirely on being exposed to a red heat. In 

 some countries, a variety which is almost entirely free from water oc- 

 curs in rocky masses, and is distinguished by the name oi Anhydrite. 



Gypsum, when burned, has ths property of being reduced with great 

 ease into the state of an impalpable powder. This powder, however, 

 combines so readily with the 21 per cent, of water it had previously lost, 

 that if it be mixed with water to the consistence of a paste so thin that it can 

 be poured into a mould, it sets and hardens in a few minutes into a solid 

 mass. In this way burned gypsum is employed in making plaster casts 

 and cornices. 



Burned gypsum consists of lime and sulphuric acid only — in the pro- 

 }.K)rtions of 41i of the former, to 58i of the latter. Its use as a manure, 

 therefore, will be specially to promote the growth of those plants by 

 which these two substances are more abundantly required, and upon 

 soils in which they are already present in comparatively small propor- 

 tion. 



7°. JSitrate of Lime. — The production of nitrate of lime in artificial 

 nitre-beds, on old walls, and on the sides of caves and cellars, especially 

 in damp situations, has already been alluded to in Lecture VIII. , [p. 

 16L] It may be formed artificially by dissolving common limestone in 

 nitric acid, and evaporating the solution. It constitutes a v bite mass, 

 which rapidly attracts water from the air, and runs to a tiquid. It is 

 produced naturally, and exists, as I believe, in soils containing lime, 

 more commonly than has hitherto been suspected. Its extreme solubili- 

 ty in water, however, renders it I'able to be carried downwards into the 

 lower portions of the soil by eveiy shower of rain — or to be actually 

 washed away, when long continued wet weather prevails. 



When heated to dull redness with vegetable matter, the nitrate of 

 lime is decomposed, and is converted into carbonate, or when exposed 

 alone to a bright red heat, the nitric acid is expelled, and quick-lime 

 alone remains. Hence where it really exists in plants, it cannot be de- 

 tected in the ash, — and when present in soils, it must be separated by 



