SILK-WORMS. r '29 
worm, as sometimes occurs in unfavourable sea- 
sons. 
Notwithstanding all this, my experiments prove 
in the ultimate analysis, that, all things balanced, 
the qualities of the soil produce but a very slight 
difference on the quality of the leaf ; that which 
will appear most evident is, that the principal in- 
fluential cause of the fineness of the silk is the de- 
gree of temperature in which the silk-worm is 
reared. I have already stated this, and I shall 
endeavour to demonstrate it in the course of the 
work. 
It is necessary to note not only the difference in 
the quality which exists in general, between the 
leaves of the mulberry proceeding from different 
soils, and picked at different seasons, but also the 
difference existing in the various sorts of mul- 
berry-trees proceeding from the same soil. 
I have experienced that, in equal proportion, 
the leaves of the broad-leaved mulberry are ra- 
ther less nutritious. 
I have observed that, after this species, the next 
is the mulberry tree that has a middle-sized leaf, 
thick, and of a dark-green colour. When these 
mulberry trees are not exposed to a dry atmo- 
sphere, and are not in light soils, they bear a large 
quantity of leaves, but they are not then of such a 
nature as to afford much silk. It appears demon- 
strated, that nature more easily produces leaves 
