3°6 Mr. Tennant on different Sorts 
whence the difference arose. For this purpose, I procured some 
pieces of each sort of limestone, and first tried what would be 
their effect upon vegetables, in their natural state, by reducing 
them to coarse powder, and sowing in them the seeds of diffe- 
rent 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 exposed for some weeks to the air, that their causti- 
city 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 penetrated to the bottom of the cup 
in which they grew. Upon examining the composition of this 
sort of lime, it proved to consist entirely of calcareous earth. 
By its exposure to the air for about three months, it was found 
to have absorbed four-fifths 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, be- 
ing formed almost entirely of the two seed-leaves, which lay 
quite loose upon the surface. This sort of lime, being spread 
upon a garden soil, to the thickness of about the tenth of an inch, 
prevented nearly all the seeds which had been sown from com- 
ing up, whilst no injury was occasioned by common lime used 
in the same manner. Upon examining the composition of this 
substance, which was so destructive to the plants, it was dis- 
covered to contain three parts of pure calcareous earth, and two 
of magnesia. The quantity of fixed air which it had absorbed, 
