386 
PHYSIOLOGY. 
BOOK II. 
1828, the effects of other gases upon plants. I much regret 
that want of space prevents my giving their experiments in 
detail : the results, which are as follows, are very important. 
Hydrochloric^ or muriatic, acid gas was found to produce 
effects not inferior, — nay, even superior, — to those of the i 
sulphurous acid. It was found that so small a quantity as a ! 
fifth of an inch, although diluted with 10,000 parts of air, ; 
destroyed the whole vegetation of a plant of considerable size ■ 
in less than two days. ‘‘ Nay, we afterwards found that a i 
tenth part of a cubic inch, in 20,000 volumes of air, had nearly i 
the same effects. In twenty-four hours the leaves of a labur- ; 
num were all curled in on the edges, dry and discoloured ; 1 
and, though it was then removed into the air, they gradually | 
shrivelled and died. Like the sulphurous acid, the hydro- 
chloric acid gas acts thus injuriously in a proportion which is 
not perceptible to the smell. Even a thousandth part of 
hydrochloric acid gas is not distinctly perceptible ; a ten- 
thousandth made no impression on the nostrils whatever, 
although great care was taken to dry thoroughly the vessels 
used in making the mixtures. 
“ Chlorine may be expected to have the effects of hydro- 
chloric acid gas ; and so indeed it has, but they appear to be 
developed more slowly. Two cubic inches, in two hundred 
parts of air, did not begin to affect a mignonette plant for three 
hours ; half a cubic inch, in a thousand parts of air, did not 
injure another in twenty-four hours: but when the plants did 
become affected, the same drooping, bleaching, and desiccation 
were observed. 
“ Nitrous acid gas is probably as deleterious as the sul- 
phurous and hydrochloric acid gases. In the proportion of a 
hundred and eightieth, it attacked the leaves of a mignonette 
plant in ten minutes; and half a cubic inch, in 700 volumes of 
air, caused a yellowish green discolouration in an hour, and 
drooping and withering in the course of twenty-four hours. 
The leaves were not acid on the surface. 
“ The effects of sulphuretted hydrogen are quite different 
from those of the acid gases. The latter attack the leaves at 
the tips first, and gradually extend their operation towards 
the leafstalks ; when in considerable proportion, their effects 
