PROFESSOR GRAHAM ON THE MOTION OF GASES. 
591 
ture, are, in regard to effusion, like different gases possessing the densities of the air 
at the two temperatures. 
As the velocity of the effusion of air does not increase at a rate so rapid as the 
direct proportion of its expansion by heat, it follows that the flow of air under pres- 
sure, through a small aperture, is retarded by heating the air ; that is, the same 
absolute quantity or weight of air will take a longer time to pass, when rarefied by 
heat, than when in a dense state. 
I have made several experiments on the influence of aqueous vapour upon the 
effusion of air. When dry air was effused into an aspirator-jar with the gauge baro- 
meter attached, and immediately afterwards air saturated with moisture at the same 
temperature, the latter passed through in sensibly the same time with comparatively 
large apertures, but in a shorter time with small apertures, although in general with- 
out much uniformity in successive experiments. Thus the time for dry air being con- 
stant at 524 seconds with plate F of small aperture, barometer 29-812, and thermo- 
meter 49° ; with moist air, the time gradually fell, till at last it appeared to settle at 
506 seconds, that number being obtained in three successive experiments ; the tem- 
perature in the mean time having risen to 51°. There is here an acceleration of 
18 seconds, of which not more than 2 seconds are accounted for by the diminished 
density of the moist air, and 1 second more by the rise in temperature. The moist 
air seemed also to have an extraordinary effect in opening and enlarging fissures, and 
very soon rendered more than one platinum plate useless, which was fixed by brazing, 
by that action. Nov. 1847-] 
Part II.— TRANSPIRATION OF GASES. 
] . Transpiration of Air of different Densities or Elasticities, by a Glass Capillary 
Tube E. 
(a.) The same arrangements were adopted as in the effusion of air of different den- 
sities, lately described, the capillary tube being interposed in the place of the per- 
forated plate. The apparatus employed is represented in fig. 4. Plate XXXIII. 
With barometer 29 28, and thermometer 54°, 65 cubic inches of dry air of the 
atmospheric density were transpired from the globular vessel a (fig. 2), into a good 
vacuum sustained by continued pumping, through a capillary glass tube E, twenty 
feet in length ; the same volume of air of 075 atmosphere and 0-5 atmosphere, 
measured at these pressures, were also transpired by the same capillary. The times 
were as follows : — 
I. 
II. 
Mean. 
Transpiration of air of 1 atmosphere 
Transpiration of air of 0 - 75 atmosphere 
Transpiration of air of 0-5 atmosphere 
799 
1049 
1545 
800 
1051 
1542 
799-5 
1050 
1543-5 
