324 
PHYSIOLOGY. 
BOOK II. 
in twelve hours ; and a third of a cubic inch in 1700 volumes 
of air affected another in twenty-four hours. The leaves 
drooped from the stem without losing colour ; and removal 
into the air, after the drooping began, did not save the plants. 
" Carbonic oxide is also probably of the same class, but its 
power is much inferior. Four cubic inches and a half, diluted 
with 100 times their volume of air, had no effect in twenty- 
four hours on a mignonette plant. Twenty-three cubic inches, 
with five times their volume of air, appeared to have as little 
effect in the same time ; but the plant began to droop when it 
was removed from the jar, and could not be revived. 
" defiant gas, in the quantity of four cubic inches and a 
half, and in the proportion of a hundreth part of the air, had 
no effect whatever in twenty-four hours. 
" The protoxide of nitrogen, or intoxicating gas, the last we 
shall mention, is the least injurious of all those we have tried; 
indeed, it appears hardly to injure vegetation at all. Seventy- 
two cubic inches were placed with a mignonette plant in a 
jar of the capacity of 500 cubic inches for forty-eight hours ; 
but no perceptible change had taken place at the end of that 
time." 
While this sheet was passing through the press, I have received Marcet^s 
paper on the effects of plants not coloured green upon the atmosphere. It 
appears from carefully conducted experiments, " that mushrooms, vegetating 
in atmospheric air, produce on that air very different modifications from 
those of green plants in analogous situations ; in fact, that they vitiate the 
air promptly, either by absorbing its oxygen to form carbonic acid at the 
expense of the carbon of the vegetable, or by disengaging carbonic acid 
formed in various ways. That the modifications which the atmosphere 
experiences when in contact with growing mushrooms are the same day and 
night. That if fresh mushrooms are placed in an atmosphere of pure 
oxygen, a great part of that gas disappears at the end of a few hours. One 
portion of the oxygen which is absorbed combines with the carbon of the 
plant to form carbonic acid ; whilst another part appears to be fixed in the 
vegetable, and to be replaced, at least in part, by nitrogen disengaged by 
the mushroom. That when fresh mushrooms remain some hours in an 
atmosphere of nitrogen, they modify very slightly the nature of that gas. 
The sole effect produced is confined to the disengagement of a small quan- 
tity of carbonic acid, and sometimes to the absorption of a very small quan- 
tity of nitrogen." 
