74 CHYMISTRY APPLIED TO AGRICULTURE. 



Gypsum carefully broken is likewise much used ; and 

 there are some farmers who attribute to it the same effica- 

 cy as is possessed by that prepared by heat. I have my- 

 self made some comparative experiments, and observed, 

 that the baked planter evidently produced a little more 

 effect the first year, but during the three years which fol- 

 lowed, the difference was almost nothing. 



The gypsum is scattered by the hand at the time when 

 the leaves of the plants begin to cover the ground, and it 

 is best to take advantage of a light rain for the operation, as 

 it is thought to be beneficial to have the leaves moistened, 

 in order that they may retain a small portion of the 

 powder. 



The effect of the gypsum is perceptible during three or 

 four years. The use of it can be resumed at the end of 

 that time. The quantity in which it is usually employed 

 is from 2f cwt. to 3^ cwt. per acre. 



Much has been said upon the effects of plaster. Some 

 have pretended that its action ought to be attributed to 

 the force with which it absorbs water. But it solidifies 

 that liquid, and does not part with it either to the atmo- 

 sphere, or to any other surrounding body; so that this 

 doctrine does not appear well founded. Besides, if its 

 action were from this cause, it would be momentary, 

 and would cease after the first rains ; and this is con- 

 tradicted by experience. Moreover, it is believed that 

 the broken gypsum has not the property of absorbing 

 water ; and yet it produces nearly the same effects as the 

 baked and powdered plaster. 



Others have thought that plaster acted only by favoring 

 the putrefaction of animal substances and the decompo- 

 sition of manures. But Davy has refuted this opinion by 

 direct experiment, placing it beyond a doubt, that the 

 mixture of plaster with manures, whether animal or vege- 

 table, does not facilitate decomposition. 



There are others, again, who attribute the effects of 

 plaster to its stimulating properties ; and these adopt, in 

 its utmost extent, the opinion which I have formed upon 

 the subject. It still remains, however, to be explained, 

 why this salt, which is not more stimulating than many 

 others, acts with so much better effect, and why its action 

 is continued during several years, whilst that of others is 

 exhausted in so much less time ; why this salt never dries 

 plants, whilst the others, if employed in excess, burn them 

 up and destroy them. These are problems which remain 



