114

DIFFUSION AND EFFUSION OF GASES.

[67.

distance of half a metre in seven minutes. On repeating the experiment with a light gas, such as hydrogen, it was found to travel downwards in the same jar about five times as rapidly as the carbonic anhydride ascended; and Graham concludes that the molecules of hydrogen in a gas apparently motionless disperse themselves to the distance of a third of a metre in a single minute (Phil. Trans. 1863, 405).

The preceding table gives the density of several important gases, the square root of the density, or ratio of the times required for the diffusion of equal volumes, if the time for air=1, the reciprocal of that square root, or calculated diffusiveness of the gas, and the actual numbers obtained by experiment, when the barometric pressure and the temperature were the same for each gas. (Graham, Phil. Mag. 1833, ii. 352.)

-

(68) Effusion.-The numbers in the last column of the table headed Rate of Effusion,' are the results obtained by experiment upon the rapidity with which the different gases escape into a vacuum through a minute aperture, about of an inch (0·086mm.) in diameter, perforated either in a thin sheet of metal or in glass (Graham, Phil. Trans. 1846, 574). It is evident that they coincide, within the limits of experimental errors, with the relative rates of diffusion of each gas; and that the velocities with which different gases pass through the same small aperture into a vacuum, are inversely as the square roots of the densities of the gases. The lightest gas enters the most rapidly. Change in the density of the gas has but little influence on the rate of effusion, the volume effised in a given time being nearly uniform, whatever the amount of condensation or of rarefaction.

The rate of the efflux of liquids, when passing through an aperture in a very thin plate, is found also to be inversely as the square roots of their densities.

On the velocity of effusion of gases Bunsen has founded an elegant method of determining their densities. The gas is introduced into a tube, from which it is allowed to escape through a minute hole in a thin platinum plate, the time occupied by the effusion of a known volume being carefully noted. The experiment is then repeated with air or some gas of known density, and from the numbers obtained the density of the first gas is easily calculated (Bunsen's Gasometry, Roscoe's translation, 1857, 121).

115

69.]

TRANSPIRATION OF GASES.

(69) Transpiration of Gases.-When gases are transmitted through fine tubes, a result very different from that furnished by diffusion is obtained, corresponding with the effect already described in the case of liquids which are allowed to escape through fine tubes. A series of experiments on gases and vapours (Graham, Phil. Trans. 1845, 591, and 1849, 349), analogous to those upon liquids by Poiseuille, already described (63), showed that the rate of efflux for each gas or the velocity of transpiration (as Graham terms this passage of gas through long capillary tubes), is entirely independent of its rate of diffusion. In the performance of these experiments, the gas was placed over

water, in a graduated jar, A, fig. 42, so suspended that the liquid in the jar and in the bath could be readily kept at the same level. The gas was dried, by causing it to pass through a tube, B, filled with calcic chloride, and was then allowed to enter through a long fine capillary tube, c, into the exhausted receiver, D, of the air-pump, which was sometimes kept vacuous by continued pumping; at other times, the state of the exhaustion was ascertained, at intervals, by means of the gauge, G. In all cases, the quantities of gas that entered in a given time were carefully observed.

It is necessary, in order to overcome the influence of effusion, and to furnish uniform results, to employ a certain length of tube, which increases with the diameter, and is not uniformly the same for all gases. If this precaution

be observed, it appears, when the gases flow through capillary tube into a

vacuum

1. That the rate of transpiration for the same gas increases, cæteris paribus, directly as the pressure; in other words, equal volumes of air, at different densities, require times inversely proportional to the densities. For example, a pint of air of double the density of the atmosphere will pass through the capillary tube into the vacuum in half the time that would be required for a pint of air of its natural density. This is a very remarkable result, and stamps the process of transpiration with a character quite unlike that of diffusion or effusion. 2. That with tubes of equal diameter, the volume transpired in equal times is inversely as the length of the tube; if 500 cubic centimetres were transpired through a tube 3 metres long, in five minutes, a similar tube, 6 metres in length, would allow the passage of only 250 cubic centimetres in the same time. 3. That as the temperature rises, the transpiration of equal volumes becomes slower. 4. That whether the tubes were of copper or of glass, or whether a porous mass of stucco were used, the same uniformity in the results was obtained. By comparing together different gases under similar circumstances, the rate of transpiration, or rapidity of passage into a vacuum through a capillary tube, was tound to vary with the chemical nature of the gas. These velocities of different gases bear a constant relation to each other, totally independent of their densities, or indeed of any other known property of the gases.

Graham considers that it is most probable that the rate of transpiration is the resultant of a kind of elasticity depending upon the absolute quantity of heat, latent as well as sensible, which different gases contain under the same volume; and therefore that it will be found to be connected more immediately with the specific heat than with any other property of gases.

Of all the gases tried, oxygen has the slowest rate of transpiration; and hence that gas may be conveniently taken as the standard of comparison for the other gases, as has been done in the preceding table, which shows the relative times in

70.]

PASSAGE OF GASES THROUGH DIAPHRAGMS.

117

which equal volumes of the different gases are transpired, and their relative velocities; these are of course inversely as the times.

A mixture consisting of equal volumes of two gases which differ in their rates of transpiration, does not always exhibit a transpirability the mean of that of the two gases when separate. The transpiration-time of hydrogen is greatly prolonged by admixture with oxygen; equal volumes of these two gases had a rate of o'9008 instead of 0.72, which would be the mean of the two.

In the following table the transpirability of some vapours is given. These results, however, from the necessity of experimenting upon the bodies in a state of mixture with some permanent gas, are not equally precise with those attained in the case of the gases above enumerated :-

Some very simple relations in the transpirability of several of the foregoing gases may be observed. Thus it has been found

1. That equal weights of oxygen, nitrogen, air, and carbonic oxide are trauspired in equal times.

2. That the velocities of nitrogen, nitric oxide, and carbonic oxide are equal.

3. That the velocities of hydrochloric acid, carbonic anhydride, and nitrous

oxide are equal.

4. That the velocity of hydrogen is double that of nitrogen, of carbonic oxide, and of nitric oxide.

5. That the velocities of chlorine and oxygen are as 3:2.

6. That the velocities of hydrogen and marsh gas are as 5:4.

7. That olefiant gas, cyanogen, and ammonia have each nearly double the velocity of oxygen.

8. That the transpiration time of hydrogen is the same as that of the vapour of ether, and that of sulphuretted hydrogen is the same as the transpiration-time of the vapour of carbonic disulphide.

Carbonic oxide and nitrogen have the same density and the same rate of transpiration; so have carbonic anhydride and nitrous oxide. The rates of transpiration of atmospheric air, oxygen, nitrogen, and carbonic oxide are likewise in direct proportion to their densities; but these seem to be concurrences rather than necessary consequences, as no regular connexion between the transpiration-time and the density of the gas can be traced.

(70) Passage of Gases through Diaphragms.-As in the case of the diffusion of liquids the results are often modified by the employment of a diaphragm, and the introduction of the disturbing force of adhesion to the material of which it consists, so it is also in respect to gases. This disturbance of the law of diffusion is strikingly seen in the case of soluble gases, when the diaphragm is moist. If a moist thin bladder, or a rabbit's stomach, be distended with air, and suspended in a jar of gaseous carbonic anhydride, the gas being soluble in the water with which the

118

PASSAGE OF GASES THROUGH DIAPHRAGMS.

[70.

membrane is wetted, is conveyed through its pores by adhesion, and passes rapidly into the inside: the air in the interior is but sparingly soluble, and is transmitted outwards very slowly; the carbonic anhydride, consequently, notwithstanding its lower diffusive power, accumulates within, and at length often bursts the bladder. A similar phenomenon, arising from the same cause, is exhibited on placing a jar of air, the mouth of which is covered by a film of soapy water, in a vessel of nitrous oxide. Where the diaphragm does not exert this solvent power, the usual law of diffusiveness prevails.

filled with air will have become convex from the endosmosis of the hydrogen; over the other it will have become concave from its exosmosis; the notion of the hydrogen in both cases through the caoutchouc being more rapid than the simultaneous passage of the air through it in the opposite direction.

It was proved many years ago by Mitchell, that caoutchouc has, like charcoal, the power of condensing large quantities of many gases by the force of adhesion; for example, it rapidly absorbs ammonia, nitrous oxide, and sulphurous anhydride. Indeed, it is impossible to employ any diaphragm in which this disturbing force is not in a certain degree observable; even with plaster of Paris it is appreciable, and slightly modifies the experimental results of diffusion :* where condensation occurs in the membrane to a large amount, the gas is frequently reduced in bulk as much as would be needed for its liquefaction; it then evaporates from the opposite surface of the diaphragm into the other gas, just as a very volatile liquid would do.

This action of colloids upon gases has been made the subject of careful investigation by Graham (Phil. Trans. 1866, 399), who regards the absorption of

* Bunsen, in his experiments (Gasometry, translation by Roscoe, 198233), used a plug of gypsum from 12 to 25mm,, or half an inch to an inch in thickness, and the results showed that the phenomena of transpiration must also be allowed for, but from estimating its importance unduly, he was led to question the accuracy of Graham's law of effusion, which is no doubt correct. An elaborate re-investigation of the whole subject by Graham has placed this beyond question (Phil. Trans. 1863, 385).