198 PROFESSOR THOMAS GRAHAm's SCIENTIFIC WORK. 



B . . . . would instantaneously recede from each other as far as possible 

 under the circumstances, and consequently arrange themselves just as in 

 a void space." At the beginning of 1803, in a supplementary paper 

 "On the tendency of elastic fluids to diffusion through each other," he 

 made known the remarkable action of intermixture which takes place, 

 even in opposition to the influence of gravity, when any two gases are 

 allowed to communicate with each other. Thus, in a particular experi- 

 ment, he showed that when a vial of hydrogen is connected with a vial 

 of carbonic gas by means of a narrow piece of tubing, so that the vial 

 of light hydrogen may be inverted over the other vial of heavy carbonic 

 gas, the heavy carbonic gas actually ascends through the light hydro- 

 gen, and the light hydrogen descends through the heavy carbonic gas 

 until the uniform admixture of the two gases with each other is efiected. 

 The subject was afterward investigated by Berthelot, who, in a series of 

 experiments performed with great care, while opposing Dalton's theo- 

 retical conclusions, corroborated his results, and indicated further the 

 high diffusiveness of hydrogen. Here it was that Mr. Graham took up 

 the inquiry. The first of his papers relating directly to the vsubject 

 of gas-diffusion appeared in the " Quarterly Journal of Science" for 

 1829, under the title, "A short account of experimental researches 

 on the diffusion of gases through each other, and their separation by 

 mechanical means."* The mode of proceeding adopted in these re- 

 searches was as follows: Each gas experimented on was allowed to 

 diffuse from a horizontally placed bottle through a narrow tube, 

 directed either upward or downward according as the gas was heavier 

 or lighter than air, so that the difiusiou always had to take place in 

 opposition to the influence of gravity. The result was that equal 

 volumes of different gases escaped in very unequal times, the rapidity 

 of the escape having an inverse relation to the specific gravity of the 

 gas. Thus hydrogen was found to escape four or five times more 

 quickly than the twenty-two times heavier carbonic gas. Again, with 

 a mixture of two gases, the lightest or most diffusible of the two was 

 found to leave the bottle in largest proportion, so that a sort of mechani- 

 cal sei)aration of gases could be effected by means of their unequal 

 diff'usibility. Most of these last results were obtained by allowing the 

 gaseous mixture to diff'use into a limited atmosi)hcre of some other 

 gas or vapor, capable of subsequent removal by absorption or condensa- 

 tion. * 



But these methods of operating, by free or adiaphragmatic dittusiou, 

 were soon abandoned by Mr. Graham for the more practicable method 

 of diffusion through porous septa. Once again, however, many years 

 afterwartl, in a paper " On the molecular mobility of gases," to be more 

 fully considered presently, Mr. Graham made some additional and very 

 curious observations on the free diffusion of hydrogen and carbonic 

 gas into surrounding air, showing the absolute velocities of the molecu- 



* Quarterly Journal of Science, ii, 1829, p. 83. 



