76 
direct experiments on its operation, when introduced 
in small quantity, and exposed to the influence of 
the solar rays. He procured seven plants of vhiea 
minor, each about eight inches high, and placed them 
in an artificial atmosphere of 290 cubic inches, 
composed of common air, mixed with J5 of carbonic 
acid. The roots of the plants were immersed in a 
little water, and the vessel that contained them was 
inverted in mercury, covered by a thin film of water, 
to prevent its noxious operation on the plants *. 
This apparatus was exposed six hours to the direct 
rays of the sun, for six days. On the seventh day, 
the plants were withdrawn, and had undergone no 
alteration : the bulk of air had not apparently in- 
creased nor diminished : neither did it exhibit any 
sign of containing carbonic acid, when washed with 
lime water. To make up, however, for the 21.75 
* M. de Saussure observes, that this effect of mercury on vege^ 
tables was first noticed by the Dutch chemists, and he found it 
always to operate when the experiment was continued for any 
length of time. In one instance, where we had confined some 
willow slips standing in the shade, .in a small bottle of water un- 
der ajar of atmospheric air, inverted over mercury, the leaves, in 
two days, were changed to a dark brown colour, and by the third 
day were quite black. This blackness extended through their 
substance, and they easily separated from the stem, though they 
had no putrid smell. The air did not contain any sensible quan- 
tity of carbonic acid, and afforded neaily its full proportion of oxy- 
gen. It was not in the least altered in bulk, for only a small glo- 
bule of mercury, not bigger than a pin's head, had gotten within 
the jar, and the lowest part of the leaves was more than three 
inches above it, which was the height of the phial, in which the 
vegetables were placed* 
