Mr. T. Graham on the Molecular Mobility of Gases. 433 



of the cavities of the stucco is added to both the air and hydrogen 

 volumes diffused ; and 3*931, 3*949, and 3*883 when such addi- 

 tion is not made to these volumes. The graphite plate, on the 

 other hand, being very thin, and the volume of its pores too 

 minute to require to be taken into account, its action is not 

 attended with the same uncertainty. With a graphite plate of 

 2 millims. in thickness, the number for hydrogen into air was 

 3*876 instead of 3*8; and for hydrogen into oxygen 4*124 

 instead of 4. With a graphite plate of 1 millim. in thickness, 

 hydrogen gave 3*993 to air 1. With a plate of the same 

 material 0*5 millim. in thickness, the proportional number for 

 hydrogen to air rose to 3*984, 4*068, and 4*067. An equally 

 considerable departure from the theoretical number was observed 

 when hydrogen was diffused into oxygen or into carbonic acid, 

 instead of air. All these experiments were made with dry gases 

 and over mercury. It appears that the numbers are most in 

 accordance with theory when the graphite plate is thick, and the 

 diffusion slow in consequence. If the diffusion be very rapid, 

 as it is with the thin plates, something like a current is possibly 

 formed within the channels of the graphite, taking the direction 

 of the hydrogen and carrying back in masses a little air, or the 

 slower gas, whatever it may be. I cannot account otherwise for 

 the slight predominance which the lighter and faster gas appears 

 always to acquire in diffusing through the porous septum. 



Inter diffusion of Gases without an intervening septum. 



The relative velocity with which different gases diffuse is 

 shown by the diffusiometer, but the absolute velocity of the 

 molecular movement cannot be ascertained by the same instru- 

 ment. For that purpose it appears requisite that a gas should 

 be allowed to diffuse into air through a wide opening. 



In certain recent experiments, a heavy gas, such as carbonic 

 acid, was allowed to rise by diffusion into a cylindrical column of 

 air, pretty much as a saline solution is allowed to rise into a 

 column of water in my late experiments upon the diffusion of 

 liquids. This method of gaseous diffusion appears to admit of con- 

 siderable precision, and deserves to be pursued further. A glass 

 cylinder of 0*57 metre (22*44 inches) in height had the lower 

 tenth part of its volume occupied with carbonic acid, and the upper 

 nine-tenths with air, in a succession of experiments : thermo- 

 meter 16° Cent. After the lapse of a certain number of minutes, 

 the upper tenth part of the volume was drawn off from the top 

 of the jar and examined for carbonic acid. Before the carbonic 

 acid appeared above, it had ascended (that is, it had diffused) 

 a distance of 0*513 metre, or rather more than half a metre. 

 After the lapse of 5 minutes, the carbonic acid so found in two 



