402 
APPENDIX A. 
on the plants beneath ; these consisted of Polypodium vulgare, Lomaria spicant^ 
Lastrcea dilatata, L. Filix-mas, Athyrium Filix-fcemina, Asplenium TrichomaneSy 
and a few other ferns, and several mosses procured from the woods in the 
neighbourhood of London, together with primroses, wood sorrel, &c. &c. Be- 
ing, however, surrounded by numerous manufactories and enveloped in their 
smoke, my plants soon began to decline, and ultimately perished, all my en- 
deavours to keep them alive proving fruitless. When the attempt had been 
given up in despair, 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, became condensed on the internal surface of the glass, 
and returned whence it came ; thus keeping the mould always in the same de- 
gree of humidity. About a week prior to the final change of the insect, a 
seedling fern and a grass made their appearance on the surface of the mould. 
I could not hut he struck with the circumstance of one of that very tribe of 
plants, which I had for years fruitlessly attempted to cultivate, coming up 
sponte suci in such a situation ; and asked myself seriously what were the con- 
ditions necessary for its growth ? To this the answer was, — Istly, an atmo- 
sphere free from soot ; (this I well knew from previous experience) : 2ndly, 
light : Srdly, heat : 4thly, moisture : and lastly, change of air. It was quite 
evident that the plants could obtain light and heat as well in the bottle as out 
of it ; and that the lid which retained the moisture likewise excluded the soot. 
The only remaining condition to he fulfilled was change of air ; and how was 
this to he effected ? When I published my account in the ‘ Companion to the 
Botanical Magazine,’ I overlooked the law respecting the diffusion of gaseous 
bodies, described in the preceding chapter, and stated that this change was 
produced by the variations of temperature causing alternate expansions and 
contractions in the air surrounding the plants, and which of course produced 
a certain but very limited effect. 
Thus, then, all the conditions necessary for the growth of my little plant 
were apparently fulfilled, and it remained only to put it to the test of experi- 
ment. I placed the bottle outside the window of my study, — a room facing 
the north, — and to my great delight the plants continued to grow well. They 
turned out to be Lastrcea Filix-mas and Poa annua. They required no atten- 
tion, the same circulation of the water continuing ; and here they remained for 
nearly four years, the Poa once flowering, and the fern producing three or four 
fronds annually. At the end of this time they accidentally perished, during 
my absence from home, in consequence of the rusting of the lid, and the ad- 
mission of rain water. Long before this occurred, however, I procured for the 
purposes of experiment some plants of Hymenophyllum and Trichomanes ; and 
perhaps the most instructive way in which I can communicate the results of my 
enquiries will be to select a few out of numberless experiments, in the order in 
which they occurred. To commence with — 
