Dialysis is a method of separating and analysing substances by means of their diffusion in alcohol or water. If a wide-mouthed vial nearly full of a solution of common salt be placed in a jar of water, after a few days it will be found that the particles of salt have come out of the vial and have diffused themselves through the superincumbent water, even to its surface. Now Professor Graham, Master of the Mint, with whom this subject originated, made three arrangements precisely like that described; the three vials were exactly similar and equal, the three jars exactly the same in size and form, and contained the same quantity of water; but the first vial contained a solution of gum arabic, the second a solution of Epsom salt, and the third a solution of common salt. After fourteen days the diffusion of the gum had risen through one half of the superincumbent water, while the particles of both the salts had risen to the surface. However the common salt would have risen much higher, for when the strata of water at the two surfaces were drawn off by a siphon and evaporated to dryness, there was fifteen times as much common salt as Epsom salt. The three solutions are heavier than water, yet they rise notwithstanding their gravitation, whence Mr. Graham thinks that there is probably an attraction between the particles of the dissolved substances and those of the water. The force of molecular attraction is more powerful than gravitation, hence the particles must rise by the difference of the two forces. After many comparative experiments the professor concluded that most substances differ in diffusibility, and that crystalloids or crystalline substances such as salts, sugars, &c., are much more diffusible than colloids or amorphous sticky bodies, such as gum, caramel, jellies, and substances that combine with the hydrogen of the water to form gelatinous hydrates. The partial decomposition of definite chemical compounds may be effected by diffusion. Alum, which is a double sulphate of the two metals potassium and aluminium, furnishes an example; when allowed to diffuse itself from its aqueous solution, the diffusive tendency of potassium compounds is so much greater than the diffusive property of aluminium compounds, that a portion of the sulphate of potassium actually breaks away from the sulphate of aluminium with which it was combined, in order to diffuse itself in the superincumbent external water more freely than the sulphate of alumina can do.