170 
ON THE DIFFUSION OF LIQUIDS. 
sidered by that acute philosopher as radically different from 
those of chemical affinity, and as the result of an attraction 
which is of a physical or mechanical kind. The characters 
indeed of these two attractions are strongly contrasted. — 
Chemical combination is uniformly attended with the evo- 
lution of heat, while solution is marked with equal con- 
stancy by the production of cold. The substances which 
combine chemically are the dissimilar, while the soluble sub- 
stance and its solvent are the like or analogous in compo- 
sition and properties. 
It was further premised, that two views may be taken of 
the physical agency by which gaseous diffusion itself is ef- 
fected, which are equally tenable, being both entirely suffi- 
cient to explain the phenomena. On one theory, that, of 
Dr. Dalton, the diffusibility of a gas is referred immediately 
to its elasticity. The same spring or self repulsion of its 
particles, which sends a gas into a vacuum, is supposed to 
propel it through and among the particles of a different gas. 
In the other theory, the existence of an attraction of the par- 
ticles of one gas for the particles of other gases is assumed. 
This attraction does not occasion any diminution of volume 
of gases on mixing, because it is an attraction residing on 
the surfaces of the gaseous molecules. It is of the same in- 
tensity for all gases ; hence its effect in bringing about inter- 
mixture is dependent upon the weight of the molecules of 
the gases to be moved by it, and the velocity of diffusion of 
a gas comes to have the same relation to its density on this 
hypothesis as upon the other. The surface attraction of 
molecules assumed,will recall the surface attraction of liquids 
which is found necessary to account for the elevation of 
liquids in tubes, and other phenomena of capillary 
attraction. 
The apparatus used in studying the diffusion of salts and 
other substances into water was very simple. It consisted 
of an open phial, to contain the solution of the salt to be dif- 
fused, which was entirely immersed in a large jar of pure 
