TRANSPIRATION, a term applied by Mr. Graham to a peculiar and fundamental property of the gaseous form of matter in passing through capillary tubes. It differs from EFFUSION by which gases pass through a small aperture about of an inch in diameter, into a vacuum ; but some of the results of transpiration correspond with the flow of liquids through capillary tubes referred to under ll/FFU SION.
The results obtained by Dr. Poiseuille, and confirmed by liegnault, were obtained by sending a liquid under examination through a capillary tube under the influence of condensed air of known preature. For a minute account of thoto experiments wo must refer to the Annales de Chitnie; 3o aerie, xxi. ; an abstract of them is also given in Professor :Miller's Chemical Physics,' where the apparatus is also fi.gured. Among the general results obtained, it appears that the rate of efflux of the liquid when the tuba exceeds a certain length (which is greeter as the diameter increases) increases directly as the pressure, so that by doubling the pressure, the amount of liquid discharged is double, the times being equal. With tubes of equal diameter the quantities discharged in equal times are inversely Al the length of the tube, so that a tube two inches long discharging 100 grains of liquid in five minutes, a similar tube four inches long would discharge only 50 grains in the same time. In tubes of equal lengths but of different diameters, the flow is as the fourth power of the diameters, so that the flow from a tube Oh of an inch in diameter would be 16 times as great as from a tuba of an inch in diameter, or as 2'. The material of the tube does not appear to influence the result ; but the nature of the liquid does so greatly. In most cases the flow of saline solutions was found to he slower than that of distilled water : the alkalies produced this effect. Certain substances appeared to exert no influence, such as nitrate of silver, corroeivo sublimate, iodide of sodium, iodide of iron, nitric, hydriodic, bromic, and hydro bromic acids. The presence of seine other substances increased the rapidity of the flow, such as bydro-sulphurio acid and hydro-cyanic acid; the nitrates of potash and ammonia, the chlorides of potassium and ammonium; the iodide, bromide, and cyanide of potassium. A slight
Increase in temperature generally augments the flow; water at 113° escaping 2j times quicker through the same tube than it did at 41°. But it is remarkable that concentrated solutions of iodide above 140° Fahr., and of nitrate of potash above 104° flow more slowly than distilled water. In general, however, the solutions contained only I per cent. of the substances ; and they were exposed to a pressure equal to that of a column of water 1 metre (39'37 inches) high, at the temperature of 52'16, and escaped through a tube 64 millimetres inches) in length, and millimetres (0.0108 inch, in diameter. No connection has been traced between the rate of efflux of the liquid and Its density, capillarity or fluidity. The dilu tion of alcohol retards its efflux, up to a certain point, beyond which it increases it, the minimum efflux corresponding with that mixture of alcohol and water which is attended with the maximum contraction. The solubility of a substance In water exerts only a secondary influence on the efflux. Dr. Poiseuille has shown that various solutions intro duced into the blood of a living animal apparently produce effects of acceleration or retardation on the capillary circulation corresponding with those noticed with the mine liquids in glass capillary tubes.
Substituting gases for liquids, it appears that the rate of efflux, or the velocity of transpiration for each gas, is independent of its rate of diffusion. In Graham's experiments on thla subject., (' Phil. Trans.' 1846 and 1849) the gas was contained in a graduated jar, standing over water, and suspended so that the water on the Inside should be kept on the amine level as that on the outside. On allowing the jar to sink, the gas was expelled by a flexible tube into a bent tube containing chloride of calcium, and being thus dried, it passed through a long capillary tube, and thus entered the exhausted receiver of an air pump, which was either kept exhausted, or the amount of exhaustion was noted by means of the gauge, the quantity of gas that: entered the receiver in a given time being carefully noted.