A. de Morgan, T. Graham, W. H. Wollaston 145 



chemical properties which may be expressed in terms of 

 molecular motion. The connexion between the density of 

 gases and the velocity of their diffusion was first investi- 

 gated by him in 1828, but established with greater precision 

 ten years later. The conclusion arrived at, that the velocity 

 of the diffusion is inversely as the square of the density, 

 proves, in the light of subsequent investigation, that the 

 molecules of different gases have at the same temperature 

 the same energy of motion. Graham's investigation 

 covered the whole field, including the inter-diffusion of 

 different gases, their transpiration through capillary tubes, 

 and their effusion into a vacuum, the peculiarities being 

 carefully examined in each case. A further series of papers 

 dealt with molecular motion in liquids, and established 

 the distinction between the inert " colloid " and the more 

 rapidly diffusing " crystalline " substances. These have had 

 important consequences, and we now know that in the col- 

 loidal state we are dealing with molecular aggregates of com- 

 paratively large dimensions, the greater individual masses 

 accounting for the slowness of the movements. Graham's 

 experiments on the passage of liquids through certain 

 membranes opened out a fruitful field of research on the 

 phenomenon called osmosis, which has recently gained 

 great importance. In the domain of pure chemistry, a paper 

 " On water as a constituent of salts " led to results of interest, 

 more especially through the discovery of the polybasic 

 nature of phosphoric acid. 



W. H. Wollaston (1766-1825), a medical man who gave 

 up his practice in order to devote himself to the study of 

 chemistry, had, in the course of his researches on platinum, 

 discovered two new elements, palladium and rhodium. 

 Investigating the peculiar power which palladium has to 

 absorb hydrogen, Graham came to the conclusion that 

 hydrogen, like a metal, could form alloys, and connecting 

 this with the chemical behaviour of this element in other 

 respects, he formed the idea that it was the vapour of a highly 

 volatile metal, to which he gave the name of " hydrogenium." 

 The expectation then raised was that hydrogen when con- 

 densed into the liquid or solid form would present the 

 characteristic appearance of a metal, but this was not 



K 



