ON MANURES. 115 



CHAPTER VI. 



MINERAL MANURES CONTINUED. GYPSUM. 



Gypsum, otherwise sulphate of lime, or plaster of Paris, 

 as it is commonly termed, consists of pure calcareous earth, or 

 lime, 30 parts or 33; sulphuric acid, 32 parts or 43; crystal- 

 lized water, 38 parts or 24. It requires from 450 to 500 

 times its own weight of \vater to dissolve it; though reducible 

 to powder in the tire, it is almost as difficult of fusion as lime- 

 stone, and it loses about 20 per cent, by calcination. When 

 pure, it does not efiervesce with acids; it is insipid in taste, 

 and free from smell; but there are other sorts which vary in 

 purity, and hence the analyses of many chemists differ in their 

 accounts of its properties. There is, however, a simple mode 

 of trying its quality, which is common in America, and con- 

 sists in putting a quantity of it pulverized into a dry pot over 

 the fire ; and when heated, it gives out a sulphurous smell. If 

 the ebullition, or bubbling, which then takes place, is con- 

 siderable, the plaster is good ; but if not, it is considered indif- 

 ferent ; and if it remains motionless, like sand, it is thought to 

 be hardly worth any thing. Another test of its goodness is 

 obtained by putting the powder alone into an iron pot over the 

 fire, and when it bubbles, like boiling water, it will admit of a 

 straw being thrust to the bottom without resistance. 



Application, (a) — When applied in its raw state, gypsum 

 is prepared for use hj first pounding it with sledge-hammers 

 into very small pieces, and then either grinding it in a mill, 

 or passing it under the crushers of oil-cake, by which about 20 

 to 25 bushels per ton are produced, according to its state of 

 purity. By the latter process, however, it is not sufficiently 

 pulverized, which is essential to its utility ; for if this be not 

 completely effected, not only will a larger quantity be required, 

 but even that will not, in some cases, be found so effectual as 

 the powder. When employed as manure, it is seldom burned, 

 and if used either in its natural state, or in proportionate 

 quantity if calcined, no perceptible difference can be disco- 

 vered, unless heavy rain should fall soon after it has been 

 spread, which gathers it into lumps, like paste, and hardens it ; 



(a) [It is now generally considered that it is best to apply gypaum in con- 

 nection with animal or organic manures.] 

 K 



