102 HOW CROPS FEED. 
keep pace with each other until all the absorbable air is 
removed from the gaseous mixture, and condensed or fixed 
in the absorbent. 
In this manner, a portion of the atmosphere enclosed in 
a large glass vessel may be perfectly freed from watery 
vapor and carbonic acid by a small fragment of caustic 
potash. By standing over sulphuric acid, ammonia is 
taken from it; a piece of phosphorus will in a few hours 
absorb all its oxygen, and an ignited mass of the rare 
metal titanium will remove its nitrogen. 
Osmose of Gases.—By this expression is understood the 
passage of gaseous bodies through membranes whose 
pores are too small to be discoverable by optical means, 
such as the imperforate wall of the vegetable cell, the 
green cuticle of the plant where not interrupted by stomata, 
vegetable parchment, India rubber, and animal membranes, 
like bladder and similar visceral integuments. 
If a bottle filled with air have a thin sheet of India 
rubber, or a piece of moist bladder tied over its mouth 
and then be placed within a bell of hydrogen, evidence is 
at once had that gases penetrate the membrane, for it 
swells outwards, and may even burst by the pressure of 
the hydrogen that rapidly accumulates in the bottle. 
Gaseous Osmose is Diffusion Modified by the Influence 
of the Membrane.—The rapidity of osmose* is of course 
influenced by the thickness of the membrane, and the 
character of its pores. An adhesion between the mem- 
brane and the gases would necessarily increase their rate 
of penetration. In case the membrane should attract or 
have adhesion for one gas and not for another, complete 
separation of the two might be accomplished, and in pro- 
portion to the difference existing between two gases as re- 
gards adhesion for a given membrane, would be the de- 
gree to which such gases would be separated from each 
* The osmose of liquids is discussed in detail in ‘‘ How Crops Grow,” p. 354, 
