S°8 Mr. Tennant on different Sorts 
described before. It was necessary occasionally to break in 
pieces the sand which had so much lime, as it would otherwise 
have been too hard to admit the seeds to penetrate through it. 
Plants will bear a much larger proportion of magnesia in ve- 
getable soil than in sand: with twenty grains, however, of 
calcined magnesia, in as much soil as was equal in bulk to four 
ounces of sand, the seeds produced only the seed-leaves, with- 
out roots ; and, with about forty grains, they were entirely 
prevented from coming up. 
In countries where the magnesian lime is employed, it was 
said, that the barrenness of any spot on which a heap of it had 
been laid, would continue for many years. To learn how far 
it could by time be deprived of its injurious qualities, I procured 
some pieces of mortar made of this species of lime, from two 
houses, one of which had been built three, and the other eight 
years : they were taken from the outside of the building, 
where they had been exposed to the air. After they were re- 
duced to powder, seeds were sown in them. Only a few came 
up, and even those produced merely the seed-leaves, without 
any roots. As plants would grow in the limestone from which 
this species of lime was formed, although not in the mortar 
made from it, I wished to know what proportion of the fixed 
air originally contained in the limestone, had been absorbed by 
the mortar. For this purpose, a piece of it was finely pow- 
dered, to render it of an uniform quality: it was then tried how 
much of this powder and of the limestone would saturate the 
same quantity of acid : by this means, I ascertained the pro- 
portions of limestone and mortar containing equal quantities 
of the magnesian lime. The fixed air being obtained from 
them in those proportions, and measured in an inverted vessel. 
