380 
POPULAR SCIENCE REVIEW. 
passage of gas into a vacuum by a small aperture in a thin 
plate ; transpiration , the passage of gases through tubes of 
fine bore and of some length ; diffusion, or the power of inter- 
penetration ; and evaporation , which is the ordinary process 
of spontaneous vapour diffusion. 
The general laws which regulate the latter alone need claim 
our attention. It is desirable to show that the ordinary process 
of evaporation is one of diffusion, and that, differing only in 
degree, it presents a striking analogy to the diffusion of gas 
through gas, and of fluid through fluid. The spontaneous 
evaporation of water into air is affected by three circum- 
stances. 1st. The dryness of the air. A certain fixed quantity 
only of water vapour can rise into air. Dr. Dalton discovered 
that the evaporation of water has the same limit in air, as in a 
vacuum. It is only necessary to know what quantity of any 
vapour rises, at a particular temperature, into a vacuum, and 
we learn the quantity which will rise into air. 2ndly. The 
influence of warmth in modifying spontaneous evaporation; 
and 3rdly, the removal of the incumbent air as it becomes 
saturated with moisture. Hence the advantages of a current of 
air for rapidly drying a wetted surface.* 
Dobereiner of Jena made the first observation on the escape 
of hydrogen through a crack in a glass receiver. He did not 
observe the whole phenomenon ; he ascribed it to capillary action, 
which indeed it resembles in some respects. It was reserved for 
Mr. Graham to clear the difficulties and give a correct interpre- 
tation of the law. In repeating Dobereiner’s experiments under 
different forms, he observed that hydrogen never escapes out- 
wards by the fissure without a certain portion of air penetrating 
inwards. Extending the experiments, and adopting an instru- 
ment which would measure the rate of the interchange, it was 
found that the diffusion of gases through fissures, or through 
porous septa, is regulated by the same law as when they freely 
communicate with each other. The relative diffusibilities are 
inversely as the square roots of the densities. 
It has now been determined that diffusion will take place 
at different rates — though always at the same rate for the 
same substance — through such porous bodies as wood, cork, 
charcoal; thin slips of many granular foliated minerals, as 
magnesian limestone ; through unglazed earthenware, slices of 
plumbago, and Carrara marble. These porous bodies may be 
regarded as a series of capillary tubes. The resistance of a 
* The papers by Dr. Dalton, and those by Professor Graham, should 
be consulted on this interesting subject; but, beyond all, “ Etudes sur 
1’IIygrometrie,” by M. Regnault, u Annales de Chimie ” (1835), should be 
studied. 
