VOL. LXXXIX.] PHILOSOPHICAL TRANSACTIONS. 54Q 



employed in agriculture, which are supposed to differ materially in their effects. 

 One of these, procured near the town, it is necessary to use sparingly, and to 

 spread very evenly over the land; as a large proportion of it, instead of increasing, 

 diminishes the fertility of the soil; and that wherever a heap of it is left in one 

 spot, all vegetation was prevented for many years. Fifty or 6o bushels on an acre, 

 were considered to be as much as could be used with advantage. The other sort 

 of lime, obtained from a village near Ferry-bridge, though considerably dearer* 

 from the distant carriage, is more frequently employed, on account of its superior 

 utility. A large quantity is never found to be injurious; and the spots which were 

 entirely covered with it, instead of being rendered barren, become remarkably 

 fertile. The different properties ascribed to these 2 kinds of lime were so very 

 distinct, that it seemed probable they could not be imaginary; and it therefore ap- 

 peared to be worth the trouble of ascertaining them more fully, and of attempting 

 to discover the nature of the ingredients from which the difference arose. For 

 this purpose, I procured some pieces of each sort of limestone, and first tried 

 what would be their effect on vegetables, in their natural state, by reducing them 

 to coarse powder, and sowing in them the seeds of different plants. In both 

 kinds the seeds grew equally well, and nearly in the same manner as they would 

 in sand, or any other substance which affords no nourishment to vegetables. 

 Pieces of each sort of stone were then burnt to lime; and after they had been ex- 

 posed for some weeks to the air, that their causticity might be diminished, some 

 seeds were sown in them. In the kind of lime which was found most beneficial to 

 land, almost all the seeds came up, and continued to grow, as long as they were 

 supplied with water; and the roots of the plants had many fibres, which had pene- 

 trated to the bottom of the cup in which they grew. On examining the compo- 

 sition of this sort of lime, it proved to consist entirely of calcareous earth. By- 

 its exposure to the air for about 3 months, it was found to have absorbed -f-ths of 

 the fixed air required to saturate it. In the other kind, a few only of the seeds 

 grew, and the plants produced from them had hardly any stalks or roots, being 

 formed almost entirely of the 2 seed-leaves, which lay quite loose on the surface. 

 This sort of lime, being spread on a garden soil, to the thickness of about T Vth 

 inch, prevented nearly all the seeds which had been sown from coming up, while 

 no injury was occasioned by common lime used in the same manner. On exam- 

 ining the composition of this substance, which was so destructive to the plants, it 

 was discovered to contain 3 parts of pure calcareous earth, and 2 of magnesia. 

 The quantity of fixed air which it had absorbed, by being exposed for about the 

 same time as the pure lime just mentioned, was only -rWths of that combined 

 with it before it was burnt. 



As it seemed probable, that the magnesia contained in this lime was the cause 

 of its peculiar properties, the following experiments were made, to determine the 

 effects of that substance on the growth of vegetables. Some seeds, chiefly of 

 colewort, which were preferred from their growing quickly, were sown in uncal- 



