GRAHAM, THOMAS (1805-1869), British chemist, born at Glasgow, was educated at Glasgow University. He was pro fessor of chemistry in the Anderson Institution, Edinburgh (1830 37), then at University College, London (183 5) , and master of the Mint (1855-69). He was F.R.S. (1836), and one of the founders of the London Chemical and the Cavendish Societies.
His first paper, published in 1826, dealt with the absorption of gases by liquids, and the first of his important memoirs on gaseous diffusion appeared in 1829. By measuring the rate at which gases diffuse through a plug of plaster of Paris, Graham developed the law, known by his name, "that the diffusion rate of gases is inversely as the square root of their density." (See DIFFUSION.) He further studied the flow of gases through fine tubes, and by effusion through a minute hole in a platinum he found that the relative rates of effusion of gases are, like their rates of diffusion, inversely proportional to the square roots of the densities.
His early work led him to examine the diffusion of one liquid into another, and as a result of the experiments he divided bodies into two classes—crystalloids, such as common salt, and colloids, of which gum arabic is a type—the former having high and the latter low diffusibility; this division has since been modified. Graham observed that in the passage through a parchment mem brane these differences still held, and so was led to devise a method —"dialysis"—for the separation of colloids from crystalloids. He also proved that the process of liquid diffusion causes partial decomposition of certain chemical compounds, the potassium sul phate, for instance, being separated from the aluminium sulphate in alum by the higher diffusibility of the former salt.