2 
PROFESSOR GRAHAM ON THE DIFFUSION OF LIQUIDS. 
with the evolution of heat, while solution is marked with equal constancy by the pro- 
duction of cold. The substances which combine chemically are the dissimilar, while 
the soluble substance and its solvent are the like or analogous in composition and 
properties. 
In the consideration of solubility, attention is generally engrossed entirely by the 
quantity of salt dissolved. But it is necessary to apprehend clearly another character 
of solution, namely, the degree of force with which the salt is held in solution, or the 
intensity of the solvent attraction, quite irrespective of quantity dissolved. In the 
two solid crystalline hydrates, pyrophosphate of soda and sulphate of soda, we see 
the same ten equivalents of water associated with both salts, but obviously united 
with unequal degrees of force, the one hydrate being persistent in dry air and the 
other highly efflorescent. So also in the solutions of two salts which are equally 
soluble in point of quantity, the intensity of the attraction between the salt and the 
water may be very different, as exemplified in the large but feeble solubility in water 
of such bodies as the iodide of starch or the sulphindylate of potash, compared with 
the solubility of hydrochloric acid or of the acetate of potash, which last two sub- 
stances are capable of precipitating the two former, by displacing them in solution. 
Witness also the unequal action of animal charcoal in withdrawing different salts 
from solution, although the salts are equally soluble ; and the unequal effect upon 
the boiling-point of water produced by dissolving in it the same weight of various 
salts. Besides being said to be small or great, the solubility of a substance has also 
therefore to be described as weak or strong. 
The gradations of intensity observed in the solvent force are particularly referred 
to, because the inquiry may arise how far these gradations are dependent upon un- 
equal diffusibility ; whether indeed rapidity of diffusion is not a measure of the force 
in question. 
I have only further to premise, that two views maybe taken of the physical agency 
by which gaseous diffusion itself is effected, which are equally tenable, being both 
entirely sufflcient 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. 
The existence of an attraction of the particles of one gas for the particles of all 
other gases is assumed in the other theory. 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 intensity for all gases, hence its 
effect in bringing about intermixture 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*. 
* Both of the molecular theories of the diffusion of gases were first publicly explained, and at the same time 
ably discussed, with the reference to the law of diffusion which had been drawn from observation, by my late 
