XXI i 



In his paper read before the Royal Society of Edinburgh, Dec. 19th, 1831, 

 he goes more fully into his favourite subject, beginning with firmness. " It 

 is the object of this paper to establish with numerical exactness the follow- 

 ing law of the diffusion of gases." " The diffusion or spontaneous inter- 

 mixture of two gases in contact is effected by an interchange in the position 

 of independent minute volumes of the gases, which volumes are not neces- 

 sarily of equal magnitude, being in the case of each gas inversely propor- 

 tioned to the square root of the density of that gas." 



In this paper (1831) he also tried the speed with which gas passed 

 through stucco under pressure. 



In 1846 Graham read to this Society a memoir "On the Motion of 

 Gases." There he showed what he called the effusion of gases into a 

 vacuum through a thin plate °f au mc ^ thick), " leaving no doubt of 

 the truth of the general law, that different gases pass through minute 

 apertures into a vacuum in times which are as the square roots of their re- 

 spective specific gravities, or with velocities which are inversely as the square 

 roots of their specific gravities," and that "the effusion-time of air of 

 different temperatures is proportional to the square root of its density at 

 each temperature." The remarkable results of transpiration are fully deve- 

 loped in his second paper (June 2 1st, 1849) "On the Motion of Gases." 



If a tube of a certain length be used to allow the escape of the gas, the 

 velocities of the gases attain a particular ratio which remains constant with 

 greater lengths and resistances. This ratio depends on a new and pecu- 

 liar property of gases, which he called Transpiration. He considered that 

 solids have many modes of showing their character, the varieties of struc- 

 ture being endless ; but gases could only show theirs in a few directions, 

 and he believed that the ratios of transpirability would have a simplicity 

 comparable to that of the specific gravities, or even the still more simple 

 relations of the combining volumes. As gases, compared with solids, are 

 capable of small variation in physical properties, those characters which 

 do show themselves may well be supposed to be the most deep-seated and 

 fundamental with which matter is endowed. He adds, " It was under this 

 impression that I devoted an amount of time and attention to the deter- 

 mination of this class of numerical constants which might otherwise appear 

 disproportionate to their value and the importance of the subject. As the 

 results, too, were entirely novel, and wholly unprovided for in the received 

 view of the gaseous constitution, of which, indeed, they prove the incom- 

 pleteness, it was the more necessary to verify each fact with the greatest 

 care." As examples, the density of nitrogen is 14 when hydrogen is taken 

 as 1 ; but the transpiration velocity of hydrogen is exactly double that of 

 nitrogen. The transpiration time of carbonic acid is inversely propor- 

 tional to its density, when compared with oxygen. These results he be- 

 lieved to show " the important chemical bearing of gaseous transpirability, 

 and that it emulates a place in science with the doctrines of gaseous 

 densities and combining volumes." 



