AND RURAL ECONOMIST. 199 



It is a geneial opinion amongst good farmers, that liming 

 should be repeated every ten or fifteen years, and that the 

 increased crops richly compensate the expense. T< matters 

 very little how it is applied, provided it is evenly spread im- 

 mediately after it is slacked. If suffered to air-slack, or to 

 lie after it has been water-slacked, it re-imbibes carbonic 

 acid, which the fire had expelled, becomes lumpy, and is 

 more difficult to be incorporated with the soil. Some spread 

 it upon the sod and plough it under, and think they have as 

 much profit from it in this way as in any other. When thus 

 applied, it powerfully contributes to decompose the tougher 

 fibres of the sod, and to convert them into nutriment for the 

 crop.' 



Gypsum, or Plaster of Paris. It is said that the pro- 

 perties of gypsum as a manure were first discovered in Ger- 

 many by a laborer at a quarry of that substance. In passing 

 across a meadow to shorten the distance home, he observed 

 the luxuriance of the grass where he had walked, and sup- 

 posing it to be caused by the dust of the gypsum from his feet 

 and clothes, made experiments, which verified his supposition. 



Great differences exist among agriculturists respecting the 

 uses of gypsum and the manner of its operation. Some 

 ha,ve supposed that its efficiency as a manure is altogether 

 owing to its power of attracting moisture from the air. But 

 Sir Humphrey Davy expressed an opinion, that but little 

 effect can be produced by such attraction. ' When com- 

 bined with water,' he observed, ' it retains that ffuid too 

 powerfully to yield it to the roots of the plant, and its ad- 

 hesive attraction for moisture is inconsiderable ; •he small 

 quantity in which it is used likewise is a circumstance un- 

 favorable to this idea.' Some have supposed that gypsum 

 assists in the putrefaction of animal substances, and the de- 

 composition of the manure in the soil. This philosopher, 

 however, proved by repeated experiments, that it rather re- 

 tards than accelerates putrefaction. He likewise says, ' In 

 examining the ashes of sainfoin, clover, and rye grass, I 

 found that they afforded considerable quantities of gypsum ; 

 and this substance probably is intimately combined as a ne- 

 cessary part of the woody fibre. If this be allowed, it is 

 easy to explain why it operates in such small quantities ; 

 for the whole of a cloved crop, or sainfoin crop, on an acre, 

 according to my estimation, would afford by incireration 

 only three or four bushels of gypsum. The reason why 

 gypsum is not generally more efficacious, is probably because 



