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Graham was born on 21 December 1805 in Glasgow, the son of a successful local manufacturer. His father had hoped that his son would, after leaving school, enter the Presbyterian ministry but in 1819, when he was only 14 years old, Graham enrolled at Glasgow University to study science. He later transferred to Edinburgh University and graduated in 1824. He returned to Glasgow to teach at the Mechanics Institute, which had been founded a year or two earlier by George Birkbeck for teaching craftsmen the scientific principles of their trades. In 1830 Graham became Professor of Chemistry at Anderson's College, Glasgow. He left Scotland seven years later to take up a similar position at University College, London, where he remained until 1854. In 1841 he became the first President of the Chemical Society of London, itself the first national society devoted solely to the science of chemistry. In 1855 he was appointed Master of the Royal Mint, a position once held by Isaac Newton. He died in London on 16 September 1869.

Graham's early interest was the dissolution and diffusion of gases. In 1826 he discovered that very soluble gases do not obey Henry's law (which states that solubility is proportional to the pressure of the gas). He measured the rates at which gases diffused through a porous plug of plaster-of-Paris, through narrow glass tubes, and through small holes in a metal plate. By 1831 he had formulated Graham' s law of diffusion, which states that the rate of diffusion of a gas is inversely proportional to the square root of its density.

In 1829 Graham turned his attention briefly to inorganic chemistry. He studied the glow of phosphorus and observed that it was extinguished by organic vapours and various gases. He went on to examine phosphorus compounds in general, particularly phosphine and salts of the various oxyacids. He distinguished ortho-, meta-and pyrophosphates, which he prepared by fusing sodium carbonate with orthophosphoric acid. Graham had made the first detailed study of a polybasic acid.

In the 1850s, following his work on gases, Graham investigated the movement of molecules in solutions. He added crystals of a coloured chemical, such as cupric sulphate, to water and noted how long it took for the colour to spread throughout the solution. He observed that different chemicals took different times to disperse and that the dispersion rate increased with increasing temperature.

Then in 1861 he tried a technique similar to that which he had used for gases. He inserted a parchment barrier across a tank of water and added a coloured salt to the water on one side of it. He discovered that some of the coloured substance passed through the barrier. Repeating the experiment using glue or gelatin, he found that these substances did not pass through parchment. All the substances tested that could pass through also formed crystals, and Graham called this category crystalloids. Those that failed to cross the barrier did not form crystals and he called these colloids (from the Greek kolla, meaning glue). He distinguished between sols and gels (although he did not use these terms to describe them).

Using the same discovery, Graham developed a method of purifying colloids. The impure colloid was placed in a porous tube suspended in running water. The crystalloids (impurities) were washed away, leaving the purified colloid in the tube. He called the process dialysis, and it has since found a multitude of applications, from desalination equipment to artificial kidney machines.

Graham maintained his interest in gases, and in 1866 started a study of the occlusion of hydrogen by metals such as iron, platinum and palladium. He observed that metal foils which freely absorb gas at low temperatures become permeable to hydrogen when heated.

Author not available, Graham, Thomas (1805-1869). , The Hutchinson Dictionary of Scientific Biography, 01-01-1998.

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