SILK-WORMS. 
171 
water and vapour that exhales from the bodies of 
the silk-worms, besides that moisture which pro- 
ceeds from the leaves, excrements, fyc. §c. 
Chapter VIII. 
OF THE REARING OF THE SILK-WORM IN THE 
LAST PERIOD OF THE FIFTH AGE ; THATIS 
TO SAY, UNTIL THE COCOON IS PERFECTED. 
Observations on the Subject. 
Let us for a while leave the silk-worms on the 
wickers, to mention some things relative to them, 
and to shew the preparations we should make for 
the accomplishment of their fifth age. 
The fifth age can only be looked on as termi- 
nated when the cocoon is perfect. When the 
silk-worm has poured out all its silk, and formed 
its cocoon, it casts its envelope, and becomes a 
chrysalis. (Chap. I.) But to form the cocoon, it 
must attain to this point, viz. of becoming a com- 
pound of only two remaining substances, the silky 
substance and animal substance.* It must then 
* Besides these animal and silky substances, which almost 
wholly compose the silk-worms, there are in their organs 
earthy particles, alkaline and acid substances, part of which 
are even in solution, as I shall hereafter shew. These sub- 
stances do not act upon each other when they arc in small 
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