

AND PHOSPHOEUS, IN LIMESTONES. 225 



the weight of the limestone. Future research will probably 

 show it to be in many cases much greater than this. In 

 burned limes the quantity of gypsum is almost invariably 

 greater than in the native limestones. The iron pyrites (sul- 

 phuret of iron) which exists in nearly all limestones, and con- 

 taminates the greater number of our coals, is decomposed in the 

 lime-kiln, and its sulphur combines in great part with the 

 burning lime. And when it is considered that inferior coals, 

 often abounding in sulphur, are generally employed in the 

 burning of lime, it will be understood that the proportion of 

 gypsum contained in burned, lime must always be considerably 

 greater than in the unburned limestones. 



Gypsum is known especially to favour the growth of certain 

 crops. It acts differently from common quicklime. Its pre- 

 sence, even in small proportion, in a large application of lime 

 cannot therefore be without effect upon the future crops. And 

 a lime which contains very little gypsum must, from this cause, 

 act differently from one which contains comparatively much. 

 This is a source of difference in the mode of action and visible 

 effects of limes and limestones from different districts, which has 

 not hitherto been made the subject of experiment. 



4. Phosphorus, or phosphoric acid in the state of phosphate 

 of lime, exists in all limestones and burned limes in minute but 

 variable quantity. From one-third of a per cent to two per 

 cent are the limits between which in burned limes the proportion 

 of phosphate of lime usually varies, according to our present 

 researches. But, though small, these quantities are sufficient 

 to produce an important influence upon the soil to which the 

 limes that contain them are applied. A ton of lime, in which 

 the phosphate of lime exists in the proportion of one-third of a 

 per cent or of two per cent respectively, contains 



When the proportion is 

 $ per cent. 2 per cent. 



Phosphate in a ton of the lime, . 7| lb. 45 Ib. 

 Equal, in common bone-dust, to . 15 ... 90 ... 



Every ton of lime in the one case, therefore, carries with it 

 as much of the phosphates as is contained in 15, and in the 

 other case as is contained in 90 lb. of bone-dust. If five tons 



