Experimental Researches in Chemistry and Physics/Escape of gases through capillary tubes

by Michael Faraday

On the Escape of Gases through Capillary Tubes^[1].

As the mobility of a body, or the ease with which its particles move among themselves, depends entirely upon its physical properties, little delay would arise in the mind, on a consideration of the probable comparative mobilities of the different gases. These bodies being nearly similar in all the physical properties, except specific gravity, which can interfere with internal motions generated in them, would be supposed to have those motions retarded in proportion as this latter character increased; but as this supposition has not been distinctly verified, the following experiments, though possessed of no peculiar claim to attention, may deserve to be recorded.

The apparatus was a copper vessel of the capacity of 100 cubic inches nearly, to which a condensing gauge was attached. Four atmospheres of the gas to be tried were thrown into it, and then a fine thermometer tube, 20 inches in length, was fixed on by adjusting pieces: the gas was suffered to escape until reduced to an atmosphere and a quarter, and the time noticed by a seconds' pendulum. In this way,—

Carbonic acid gas	required	156·5	minutes	to escape.
Olefiant gas	„	135·5	„	„
Carbonic oxide	„	133	„	„
Common air	„	128	„	„
Coal-gas	„	100	„	„
Hydrogen	„	57	„	„

These experiments tend to show, that the mobility of the gases tried decreases as their specific gravity increases, and they are corroborated by others made with vanes. A wheel, having small planes attached to it, as radii perpendicular to the plane of motion, was made to rotate by a constant force in atmospheres of different gases, and the times which the motion continued, after the force was removed, diminished as the specific gravity increased; as for instance, in

Carbonic acid	it continued	6	seconds.
Common air	„	8	„
Coal-gas	„	10	„
Hydrogen gas	„	17	„

There is therefore every reason to believe, that the actual relative mobilities of the gases are inversely as their specific gravities.

These experiments have been carried much further, in consequence of some peculiar results obtained at low pressures; but as I have not been able to satisfy myself respecting the causes, and have probably taken a wrong view of the phenomena, I shall refrain from detailing them, and merely observe, that there is no apparent connexion between the passage of gases through small tubes and their densities at low pressures. Olefiant gas then passes as readily as hydrogen, and twice as rapidly as either carbonic oxide or common air, and carbonic acid escapes far more readily than much lighter gases. Similar results are also obtained by diminishing the bore of the tube, and then even at considerable pressures, the effect produced by mobility alone is interfered with by other causes, and different times are obtained. These anomalies depend, probably, upon some peculiar loss or compensation of forces in the tube, and offer interesting matter of discussion to mathematicians.

1. Quarterly Journal of Science, iii. 354.