AGRICULTURAL CHEMISTRY AND GEOLOGY. 171 



of nitrogen, and finds in the vegetation of this plant a convincing proof that the gaseous 

 nitrogen of the atmosphere is not assimilated by plants. Comptes Rendus, Oct. 2, 1854, p. 601. 



On the Agricultural Value of Gypsum. 



BY SAMUEL W. JOHNSON. 



IT is nearly one hundred years since gypsum (plaster of Paris, sulphate of lime) began to 

 acquire its agricultural significance. Since that time it has become celebrated on account of 

 the successes that have attended its use ; while the ill results, and want of results, that have 

 followed its application, have not failed to make it many enemies. Franklin, in the United 

 States, and Schubert Von Kleefeld,* in Germany, towards the close of the last century, 

 simultaneously gave a great impulse to the use of gypsum. Within the last seventy years, an 

 immense number of observations and experiments have been made with it ; and yet to this 

 day the method and condition of its action are very imperfectly understood. 



Before we attempt to learn that which is now unknown, we must first of all carefully ex- 

 amine our existing stores of knowledge ; we must have in mind all that has been done and 

 learned relating to the subject : we thus acquire points of departure, discover the trails which 

 may guide us through the maze, and save ourselves the trouble of repeating what has been 

 already either well or vainly done. 



What is gypsum f When pure and unburned, 100 pounds contain 



Lime 32J pounds. 



Sulphuric acid 46* " 



Water... 21 " 



100 < 



The water is in chemical combination with the sulphate of lime. By heating the com- 

 pound, the 21 per cent, of water is driven off; and what remains, called burned or boiled 

 plaster, consists in 100 parts of 



Lime 41 parts. 



Sulphuric acid 59 " 



100 



The agricultural effect of burned and unburned plaster, so far as we know, is precisely alike ; 

 for when the former is exposed to dews or rains, it immediately recovers its water, unless it 

 has been too strongly heated, in which case it attracts water slowly or not at all. This at- 

 traction of water is in itself no advantage, for the water attracted combines chemically with 

 the plaster, and can never be of avail to the plant any more than the water already contained 

 in xinburned gypsum. When the plaster has thus satisfied its chemical thirst for water, it 

 has no more absorbent power for that substance than so much ordinary soil ; and hence the 

 notion that plaster helps vegetation to water, and is thus of agricultural value, is not sup- 

 ported by a particle of evidence. The advantages of burned plaster are, that it is more easily 

 reduced to a fine powder, which facilitates its solution in water and its distribution through- 

 out the soil. Besides, by its use the transportation of 21 per cent, of water is saved. On the 

 other hand, unburned plaster is cheaper by so much as the burning costs ; and burned plas- 

 ter, if too strongly heated, may become a little less readily soluble in water. This latter con- 

 sideration is not probably of much weight, so that it is reasonable to suppose that on the soil 

 79 pounds of plaster = 100 pounds of unburned plaster. Actual experiments have failed to 

 establish any superiority in the agricultural effect of one form over the other, in case both 

 were equally pulverized. 



The above statements refer to pure sulphate of lime ; but plaster, as quarried for agricul- 

 tural purposes, often contains several per cent, of admixture, as clay, carbonate of lime, &c. 

 These are of little consequence unless their quantity be quite considerable. The presence of 

 quick-lime in calcined may perhaps account for the ill success of some in fixing ammonia 



* Schubert of Clover-field, so knighted by Joseph I., on account of his merit in extending the cultivation 

 of clover. 



