36 THE IMITATION, IN CLOSED CASES, OF 
obtain this end, I built np some rock-work 
in the yard at the back of my house, and placed 
a perforated pipe at the top, from which water 
trickled on the plants beneath ; these consisted 
of Polypodium vulgare , Lomaria Spicant, Lastrcea 
dilatata, L. Filix mas, Athyrium Filix fcemina, 
Asplenium Triehomanes and a few other ferns, 
and several mosses procured from the woods in 
the neighbourhood of London, together with 
primroses, wood -sorrel, &c. In consequence, 
however, of the volumes of smoke issuing from 
surrounding manufactories, my plants soon began 
to decline, and ultimately perished, all my endea- 
vours to keep them alive proving fruitless. 
When the attempt had been given up in 
despair, a fresh impetus was given to my pursuits, 
and I was led to reflect a little more deeply upon 
the subject, in consequence of a simple incident 
which occurred in the summer of 1829. I had 
buried the chrysalis of a sphinx in some moist 
mould contained in a wide-mouthed glass bottle, 
covered with a lid. In watching the bottle from 
day to day, I observed that the moisture which, 
during the heat of the day arose from the mould, 
condensed on the surface of the glass, and re- 
turned whence it came; thus keeping the earth 
always in the same degree of humidity. About 
a week prior to the final change of the insect, 
