4, On Lime. 
a SE 
effect. From this 1 conclude that lime must act as a 
stimulant, and that the quantity applied to the land 
ought to bear an exact proportion to the carbonic, or 
vegetable matters contained in the soil. It is well 
known that stimulants used in small quantities are in 
some cases very useful in the animal economy, but in 
great quantities they will destroy animal life. 
I would not by any means presume to. call in ques- 
tion the results of Mr. 'Tennant’s or Dr. Black’s expe- 
riments ; on the contrary, I am rather inclined to be- 
lieve, that all stone lime contains a greater or lesser 
proportion of magnesia; but that the ime which con- 
tains the greatest proportion of that earth, is totally un- 
fit to be used upon land as a manure, I think, deserves 
a second consideration. 
Dr. Darwin observes that the substance called chalk- 
stone is almost wholly magnesia; now I. know from 
experience that chalk-stone land is the most kindly to 
all sorts of grain of any soil I am acquainted with, and 
will bear a longer succession of severe cropping before 
it is exhausted. But perhaps it is the process of calci- 
nation which gives to the magnesia that caustic quality 
which renders it so hurtful to vegetation, as Mr. Ten- 
nant found by his experiments, that thirty or forty 
grains of lime did not retard the growth of seeds, more 
than three or four of calcined magnesia. From which 
Dr. Darwin concludes, that, as both injure vegetation 
in large quantities, they may both assist vegetation in 
small ones. 
Consistent with the Doctor’s remark I would just ob- 
serve, that there are many substances which make rich 
~ manures, when used in small quantities ; for instance 
