100 HOW CROPS FEED, 
which compose the atmosphere are uniformly mixed to- 
gether without regard to their specific gravity, is but one 
result of a law of nature which we shall immediately 
notice. 
Diffusion of Gases.—Whenever two or more gases are 
brought into contact in a confined space, they instantly 
begin to intermingle, and continue so to do until, in a 
longer or shorter time, they are both equally diffused 
throughout the room they occupy. If two bottles, one 
filled with carbonic acid, the other with hydrogen, be con- 
nected by a tube no wider than a straw, and be placed so 
that the heavy carbonic acid is below the fifteen times 
lighter hydrogen, we shall find, after the Japse of a few~ 
hours, that the two gases have mingled somewhat, and in 
a few days they will be in a state of uniform mixture. On 
closer study of this phenomenon it has been discovered 
that gases diffuse with a rapidity proportioned to their 
lightness, the relative diffusibility being nearly in the in- 
verse ratio of the square roots of their specific gravities.. 
By interposing a porous diaphragm between two gases of 
different densities, we may visibly exhibit the fact of their 
ready and unequal diffusion. For this purpose the dia- 
phragm must offer a partial resistance to the movement 
of the gases. Since the lighter gas passes more rapidly 
into the denser than the reverse, the space on one side of 
the membrane will be overfilled, while that on the other 
side will be partially emptied of gas. 
In the accompanying figure is represented a long glass 
tube, 4, widened above into a funnel, and having cemented 
upon this an inverted cylindrical cup of unglazed porce- 
lain, a. The funnel rests in a round aperture made in the 
horizontal arm of the support, while the tube below dips 
beneath the surface of some water contained in the wine- 
glass. The porous cup, funnel, and tube, being occupied 
with common air, a glass bell, ¢, is filled with hydrogen 
gas and placed over the cup, as shown in the figure, In. 
