352 
PHYSIOLOGY. 
BOOK II. 
till they die. Solutions of arsenic and arseniate of potash 
produce the same effects. 
A leaf of the sensitive plant was in a cold diluted solution 
of opium : in a few moments it opened out as in water, and, 
* after half an hour, gave the usual signs of contractibility. In 
six hours it was expanded, and had a natural appearance, but 
could not be excited to move. The leaflets were flexible at 
the articulations, and offered a singular contrast to the state 
of irritation produced by corrosive sublimate. Pure water did 
not recover the plant. A large branch, similarly situated, 
expanded its leaves ; but in half an hour had lost much of 
its sensibility : the leaflets, though alive, seemed asleep, and 
required much stimulating to cause contraction. In one hour 
the contractions ceased : in two hours the branch was dead. 
A leaf placed in prussic acid (Scheele's strength) con- 
tracted, then slightly dilated, but was quite insensible, and 
the articulations were flexible : water did not recover it. If 
the acid be very weak, the leaflets dilate and appear to live, 
but are insensible. A drop of the acid plaeed on two leaflets 
of a healthy plant gradually causes contraction of the other 
leaflets, pair by pair. Solutions of opium and corrosive 
poisons have no effect when applied this way. After some 
time they dilate, but are insensible to external irritation : the 
sensibility returns in about half an hour; but the leaflets 
appear as if benumbed. 
The plant exposed to the vapour of prussic acid is affected 
in the same way : ammonia appears to favour the recovery 
of the plant. 
A cup containing dilute prussic acid was so placed that 
one or two leaves, or sometimes a branch, of a healthy plant 
could be plunged into the liquid, or left to repose on its sur- 
face. The leaflets remained fresh and extended, but were 
almost immediately insensible. Being left in this state for 
two hours, they were expanded ; and no irritation could cause 
their contraction, though otherwise ^there was no appearance 
of an unnatural state. At five o'clock in the evening the 
leaves were left to themselves. At nine o'clock they were 
open and insensible. At midnight they were still open, 
