THOMAS GRAHAM, (1804-1869), born at Glasgow on the 21st of December 1804, was the son of a merchant of that city. In 1819 he entered the university of Glasgow, and graduated in 1824. At this time the chair of chemistry was held by Dr Thomas Thomson, whose researches bear-ing on the atomic theory cannot fail to have had much influence in turning Graham's thoughts to the study of molecular physics to which he so patiently devoted his life. The beginning of his career appears to have been much embittered by his father's opposition, who wished him to become a minister of the Established Church. His own views, however, prevailed, and he worked for two years in the laboratory of Dr Hope of Edinburgh before returning to Glasgow, where he taught mathematics, and subsequently chemistry, until the year 1829, when he was appointed lecturer in the Mechanics' Institute. In 1830 he succeeded Dr Ure as professor of chemistry in the Anclersonian Insti-tution, and, on the death of Dr Edward Turner, he was transferred to the chair of chemistry in University College, London. He presided over the chemical section of the British Association at the Birmingham meeting in 1839, and in 1841 was chosen as the first president of the Chemi-cal Society of London. He resigned his professorship on being appointed to succeed Sir John Herschel as Master of the Mint, a post he held until his death in September 1869. This appointment was doubtless offered to him by Govern-ment in recognition of his scientific services, but the onerous duties of the important office severely tried his energies; and it is unfortunate that, in quitting a purely scientific career, he should have been subjected to the cares of official life for which he was by temperament singularly unfit. The researches, however, which he conducted between 1861 and 1869 were as brilliant as any of those in which he engaged. Graham was elected a fellow of the Royal Society in 1837, a corresponding member of the Institute of France in 1847, and doctor of civil law in 1855. The presidency of the Royal Society was offered him towards the close of his life, but his failing health caused him to shrink from accepting the honour.
The persistency with which he traced and developed the laws of atomic motion was remarkable. It is interesting therefore to remember that his future work must have been indicated in no small measure by the researches of the illustrious Black, who, at the beginning of the century, rejected the definitions of chemistry proposed by Stahl, Boerhaave, and Fourcroy, and lectured " on the effects produced by heat and mixture in all bodies or mix lures of bodies natural or artificial." Graham communicated papers to the Philosophical Society of Glasgow before the work of that society was recorded in Transactions, but his first published paper, "On the Absorption of Gases by Liquids," appeared in the Annals of Philosophy for 1826, and is of special interest, as in it he speaks of the liquefaction of gases in much the same terms as those employed in the last paper he wrote. The subject with which his name will always be most prominently associated is the molecular mobility of gases. Priestley observed in 1799 that hydrogen escaped from a fissured glass jar in exchange for external air which " had nothing inflammable in it," and Dalton proved in 1806 that gases confined in glass phials, connected by glass tubes, intermix even against the action of gravity. Graham in his first paper on this subject (1829) thus summarizes the knowledge experiment had afforded as to the laws which regulate the movement of gases. " Fruitful as the miscibility of gases has been in interesting speculations, the experimental infor-mation we possess on the subject amounts to little more than the well-established fact that gases of a different nature, when brought into contact, do not arrange themselves according to their density, but they spontaneously diffuse through, each other so as to remain in an intimate state of mixture for any length of time." For the fissured jar of Priestley and Dobereiner he substituted a glass tube closed by a plug of plaster of Paris, and with this simple ap-pliance he developed his now well-known law " that the diffusion rate of gases is inversely as the square root of their density."
With regard to the special importance of Graham's law to the chemist and physicist, it may be sufficient to point out that a great number of chemical as well as physical facts are co-ordinated by the assumption that all substances in the state of gas have the same molecular volume or contain the same number of molecules in a given space (Avogadro's law); and, in the second place, it has become evident that the phenomena of heat are simply the manifestations of mole-cular motion. According to this view the absolute tempera-ture of a gas is proportional to the vis viva of its molecules ; and since all molecules at a given temperature have the same vis viva, it follows that the molecules must move with velocities which are inversely proportional to the square roots of the molecular weights. Moreover, since the mole-cular volumes are equal, and the molecular weights are therefore proportional to the densities of the aeriform bodies in which the molecules are active units, it also follows that the average velocities of the molecules in any two gases are inversely proportional to the square roots of their respective densities. Thus the simple numerical relations first ob-served in the phenomena of diffusion are the direct result of molecular motion, and it is now seen that Graham's empirical law is included under the fundamental law of motion.
Graham also studied the passage of gases by transpiration through fine tubes, and by effusion through a minute hole in a platinum disc, and was enabled to shovv that gas may enter a vacuum in three different ways : (1) by the molecular movement of diffusion, in virtue of which a gas penetrates through the pores of a disc of compressed graphite ; (2) by effusion through an orifice of sensible dimensions in a platinum disc (the relative times of the effusion of gases in mass being similar to those of the molecular diffusion, although a gas is usually carried by the former kind of im-pulse with a velocity many thousand times as great as is demonstrable by the latter); and (3) by the peculiar rate of passage due to transpiration through fine tubes, in which the ratios appear to be in direct relation with no other known property of the same gases,thus hydrogen has exactly double the transpiration rate of nitrogen, the relation of those gases as to density being as 1 : 14.
He subsequently examined the passage of gases through septa or partitions of india-rubber, and plates of non-crystalline metals such as palladium, and proved that gases pass through these septa neither by diffusion, effusion, nor transpiration, but in virtue of a selective absorption which the septa appear to exert on the gases in contact with them. By this means he was enabled partially to separate oxygen from air, and to calculate the density of metallic-hydrogen from the remarkable expansion which attends the absorption of hydrogen by palladium, The experiments led him to believe that palladium with its occluded hydrogen was an alloy, a view that has been greatly strengthened by the recent experiments of MM. Cailletet and Pictet.
His early work on the movements of gases led him to examine the spontaneous movements of liquids, 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, He also proved, by a series of beautiful experiments, that the process of liquid diffusion actually causes partial decomposition of certain chemical compounds, the sulphate of potash, for instance, being separated irom tne sulphate ot alumina in alum by the higher diffusibility of the former salt.
He also extended his work on the transpiration of gases to liquids, adopting the method of manipulation de-vised by Poiseuille. He found that dilution with water does not effect proportionate alteration in the transpiration velocities of different liquids, and a certain determinable degree of dilution retards the transpiration velocity. Thus in the case of alcohol the greatest retardation is with six equivalents of water, nitric acid with three, and acetone with as much as twelve equivalents.
It is only possible here to indicate the prominent features of Graham's more purely chemical labours. In 1833 he showed that the various compounds of phosphoric acid and water constitute distinct salts, in each of which the hydrogen may be displaced by other metals. He was the first, there-fore, to establish the existence of polybasic compounds, in each of which one or more equivalents of hydrogen are replaceable by certain metals, and he further showed that by heating biphosphate of soda a metaphosphate is formed, and from this he obtained a corresponding hydrated acid. In 1824 he demonstrated that the spontaneous inflamma-bility of one variety of phosphuretted hydrogen is due to its admixture with a very small proportion of an oxide of nitrogen, probably nitrous acid. In 1835 he published the results of an examination of the properties of water as a constituent of salts. Kot the least interesting part of this inquiry was the discovery of certain definite salts with alcohol analogous to hydrates, to which the name of alco-holates was given. A brief paper entitled Specidative Ideas on the Constitution of Matter deserves notice as possessing special interest in connexion with work done since Graham's death. In it he expressed the view that the various kinds of matter now recognized as dif-ferent elementary substances may possess one and the same ultimate or atomic molecule in different conditions of move-ment.
Graham's work, viewed as a whole, is remarkable alike for its originality and for the singular simplicity of the methods employed in obtaining most important results.
Biographical notices of Graham will be found in the Proceedings of the Royal Society, xviii., 1870, p. xviii. ; Proceedings of the Royal Society of Edinburgh, vii., 1872, p. 15 ; Proceedings of the Royal Institution, vi., 1872, p. 15; Deutsch. Chem. Gesellschaft, Berlin, ii., 1869, p. 753; München Alcad. Sitzungsb., 1870, i., p. 408; American Journal of Science, i., 1871, p. 115; Smithsonian Reports, 1871, p. 177; Proceedings of American Academy, viii., 1870, p. 230. His works have been collected and printed by Dr James Young and Dr Angus Smith, the latter contributing to the volume a valuable preface and analysis of its contents. (W. C. R.)