By employing a certain length of tube increasing with the diameter, not the nine for all gases, it appears that the rate of transpiration increases directly as the pressure, so that equal volumes of air at different densities require times Inversely proportioned to the densities : —thus a pint of air double the density of the atmosphere will less through the capillary tube into vacuum in half the time required for a pint of air of ordinary density. With tubes of equal bore the volume transpired in equal times is inversely as the length of the tube; thins, if 30 cubic inches of gas were transpired through a tube 10 feet long in 5 minutes, a similar tube 20 feet long would only allow the passage of 15 cubic inches in the same time. It was also found that the transpiration of-equal volumes becomes slower as the temperature rises. Uniform results were also obtained, whether the tubes were of copper or of glass, or a porous mass of stucco were employed. Trans piration was found to vary with the chemical nature of the gases. The velocities of transpiration of different gases bore a constant relation to each other, independently of their densities, and it was thought probable that the rate of transpiration is the resultant of a kind of elasticity depending on the absolute quantity of heat, latent as well as sensible, which different gases contain under the same volume, so that transpiration seems to be intimately connected with the specific heat of aaes.
has apparently the 'lowest rate of transpiration, and is taken as the unit in the following table. It is found that a mixture of equal volumes of two gases does not always give the mean transpim bility. Thus the time for the transpiration of hydrogen is much prolonged by admixture of oxygen, the rate being instead of the mean In these experiments capillary glass tubes, varying from 20 feet to 2 inches in length, gave similar results, where a sufficient resistance was offered to the passage of the gas. The effusion of gases, or the discharge by an aperture in a thin plate, is dependent in all gases upon a constant function of their specific gravity ; but the discharge of the same gases from tubes has no uniform relation to the density of the gases. Both hydrogen and carbonic acid, for example, jai% more quickly through a tube than oxygen, although the one is lighter and the other heavier than that gas.
One of the capillary tubes used by Graham was as much as 22 feet in length ; it was made up of several portions of capillary tube as nearly equal in bore as could be judged of by the eye, cemented together by the blow-pipe so as to form a continuous length, but bent up into coils for the convenience of using. Its extremities were connected with block-tin tubes, proceeding from the drying-tube and air-putnp jar by means of thick caoutchouc adopters. This long capillary tube allowed one cubic inch of air to pass into a vacuum in seconds ; two inches of the tube held grains of mercury, which gives a diameter of 0'0222 inch, or of an inch.
It was found possible to forma capillary tube of copper of legs dia meter than one of glass, by the following means :—a cylindrical hole was first drilled in the axis of a solid copper rod, four or five inches long, which rod was then extended by passing it through a wire draw. plate. An iron wire, or triplet, was placed within the copper tube, and drawn through the plate at the same time, in order to keep the surface of the copper tube smooth and uniform. It was found necessary to pull out the iron wire every time the copper was drawn, to prevent its becoming fixed. The iron wire was then extended separately, and again introduced into the copper tube, and the operation of drawing out was repeated. In this way the copper tube was extended 11 feet 8 inches, and it remained perfectly sound and airtight. One cubits inch of air passed through it into e vacuum In 22.12 seconds. Its diameter was thus found :—Of the iron wire on which the copper was last drawn, 921 inches weighed grains, or ono inch weighed grains. Taking the specific gravity of iron at 71, this gives the diameter of the copper tube 0.0114 inch, or hth of an inch. In using this tube for transpiration experiments, It was coiled up into circles about 10 inches in diameter.
Mr. Graham's experiments, which are exceedingly numerous, are very neat and precise in their results : the experiments of Dr. Poiseuille have also an equal constancy and precision of result in the passage of liquids through capillary tubes which is quite remarkable. To take, for example, a few of Mr. Graham's results : the transpiration velocity of hydrogen is exactly double that of nitrogen, although the relation in density is as 1 : 14. The transpiration of carbonic oxide, like the sp. gr., is also identical with that of nitrogen. The transpira tion velocity of oxygen is related to that of nitrogen in the inverse ratio of the densities of these gases, that is, as 14 : 16. In equal times, and with equal weights (not equal' volumes) of these two gases, the more heavy gas was more slowly transpired in proportion to its greater density. Mixtures of oxygen and nitrogen have the mean velocity of these two gases, and hence the time of air is also found to be propor tional to its density when compared to the time of oxygen. Indeed, the velocity of different gases through capillary tubes, bears a constant relation to each other. The constancy of these relations, or of the transpiration times, has been observed for several of the gases for tube resistances, varying in amount from 1 to 1000. These relations are more simple in their expression than the densities of the gases. It is, indeed, very remarkable to find the velocity of hydrogen to be exactly double that of nitrogen and carbonic oxide ; the velocity of nitrogen and oxygen to be inversely as the specific gravity of these gases; the velocity of binoxide of nitrogen to be the same as nitrogen and carbonic oxide ; the velocity of carbonic acid and nitrous oxide to be equal and directly proportional to their specific gravities when compared with oxygen. In like manner, it is found that the velocity of proto-carbu retted hydrogen is hydrogen being = I, the velocity of chlorine is 1 that of oxygen, of bromine vapour and sulphuric acid vapour the same as oxygen, while that of ether vapour is the same as hydrogen. Olefiant gas, ammonia, and cyanogen are equal, or nearly equal, in velocity which approaches closely to double that of oxygen. Hydro sulphuric acid gas and the vapour of bi-sulphide of carbon have an equal transpiration time. The compounds of methyl have a less velo city than the corresponding compounds of ethyl, but are connected by a certain constant relation.
Among the general results obtained by Mr. Graham are the fol lowing :-2. That the resistance of capillary tubes of uniform bore to the passage of any gas is directly proportional to the length of the tube. 8. That the velocity of the passage of equal volumes of air of the same temperature, but of different densities or elasticities, Is directly propor tional. 4. That rarefaction by heat has a precisely equal effect in diminishing the velocity of the transpiration of equal volumes of air, as the loss of density and elasticity by diminished pressure has. 5. That a greater resistance In the capillary tube is required to bring out the third result, or law of densities, than is apparently necessary for the first or second result, and a resistance still further increased, and the highest of all, to bring out the fourth result, or the law of tempera tures. 6. That transpiration is promoted •by density, whether due to compression, to cold, or to the addition of an element in combination, as the velocity of oxygen is increased by combining with carbon, with out change of volume in carbonic acid.
With respect to the influence of transpiration on the distribution of coal-gas by means of pipes, the results are similar with truly elastic gases whether the tubes be capillary or many inches in diameter, pro vided the length of the tube be not less than 4000 times its diameter. The small propulsive pressure applied to coal-gas is favourable to transpiration as well as the great length of the mains. The velocity of coal-gas should be air being 1 under the same pressure. With a constant propulsive force in the gasometer, the flow of gas should increase in volume with a rise of the barometer or with a fall in tem perature directly in proportion to the increase of its density from either of these causes.