112 COMPOSITION OF LIME. 



is these from which the hydraulic or water cement is 

 made. Although magnesia is necessary to plants, 

 caustic magnesia, if introduced in large quantity into 

 the soil, seems to produce a very bad effect, and lime 

 that contains much of it is therefore to be avoided. 



Beside limestones, there are several other forms in 

 which lime is largely used by the farmer. The chief 

 of these is marl. This substance consists usually of 

 the fragments and dust of sea, fresh-water, or land 

 shells, more or less mixed with earth. When pure, 

 the greater proportion is carbonate of lime. The fol- 

 lowing table gives the composition of a very excel- 

 lent one, lately analyzed in my laboratory. It was 

 from Peterboro', N. Y.: 



TABLE vn. 



Iba. in 100. 



Carbonic acid, 35*00 



Lime, 45*02 



Magnesia, 0*66 



Iron and alumina, with a little phosphoric acid, . 2"69 



Sand, 9-57 



Organic matter, 7'06 



100-00 



Here the carbonate of lime amounts to about 80 

 lbs. in 100, while the small quantities of magnesia, 

 iron, alumina, and especially of phosphoric acid, add 

 materially to its value. There are many marls which 

 do not contain more than from 15 to 25 per cent of 

 lime. It is necessary to apply these in much larger 

 quantity, to produce an equal effect, and of course 

 they will not bear transportation to so great a dis- 

 tance. In using marls, it is always best to put on 

 heavier doses than of any form of burned lime, as 

 there is not, from its mild nature, the same risk of 

 add i no- too much. 



