THE NATURAL CONDITIONS OF PLANTS. 29 
all gases pass through, membranous septa, they 
differ remarkably in the relative rapidity of trans- 
mission. Thus, while a volume of carbonic acid 
requires five and a half minutes to pass through 
a membrane, the same volume of oxygen requires 
one hundred and thirteen, and a much greater 
time is required for nitrogen. Hence, when a 
bladder full of air is surrounded by carbonic acid, 
the latter enters faster than the former escapes, 
and the bladder bursts ; but on reversing the con- 
ditions of the experiment, the bladder becomes 
flaccid, because the carbonic acid within passes 
out more rapidly than the exterior air enters. 
The transmission of gases in some of these expe- 
riments takes place in opposition to a pressure 
equal to several atmospheres.” 
To conclude this curious subject, Spallanzani 
proved that some animals possessed of lungs, — 
such as serpents, lizards, and frogs, — produce the 
same changes on the air by means of their skin, 
as by their proper respiratory organs; and Dr. 
Edwards, in a series of masterly experiments, 
has shown that this function compensates so fully 
for the want of respiration by the lungs, as to 
enable these animals, in the winter season, to live 
for an almost unlimited period under the surface 
of the water. 
“ It is scarcely possible,” says Professor Daniel, 
