Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/766

Rh 730 E V A P O li A T I N matter, and the results at which he arrived are perfectly conclusive within the errors of his experiments. First he measured the pressure of a quantity of dry air kept at constant volume for every degree on Fahrenheit s scale between the freezing and boiling points ; then he found the pressure of pure steam in contact with water for every degree through the same range, and lastly the rate of increase of pressure of a quantity of air kept at constant volume but in contact with water when the temperature varied. The results showed that at each particular temperature the pressure of the air saturated with vapour was exactly equal to that corresponding to the dry air together with that exerted by vapour alone when in contact with water at the same temperature ; from which he inferred that there is either no chemical action between the air and vapour, or such action in no way affects the question at issue with gases other than air and vapours other than aqueous. This conclusion is frequently ex pressed by saying that gases and vapours behave to one another as vacua. Most of these experiments were published in a paper in the Manchester Memoirs, vol. v. Dalton was the first to give a table of the maximum pressure of steam for temperatures from 80 to 212 Fahr. The researches of Desormes, Gay Lussac, and Daniell alt tend to corroborate Dalton s theory and the accuracy of his experiments, the results of which may be summed up in two statements, sometimes cited as Dalton s laws, viz. : i. In a space which contains a liquid and its vapour only, the liquid will continue to evaporate until the pressure of its vapour attains a determinate amount depen dent only on the temperature. II. In a space containing dry air or other gas or gases a liquid will continue to evaporate until the pressure exerted by its vapour alone ia the same as if no air or other gas were present. The more recent researches of Regnault and Andrews have shown that the second law is not quite true. It was, however, a great step in advance, and is sufficiently accurate for all ths purposes of chemical analysis and hygrometry. Two or more vapours will act towards one another as vacua when, and only when, their liquids have no affinity for one another. &quot;When this is not the case, the pressure exerted by the vapour above the surface of the mixed liquids is frequently much less than that which can be exerted by the vapour of the more volatile liquid alone. Thus sulphuric acid will absorb aqueous vapour, and alcohol will absorb ether vapour, reducing the pressure to a small fraction of that exerted by the ether vapour alone. Bisulphide of carbon and paraffin oil also diminish the pressure of ether vapour. Since a mixture of liquids may boil when the pressure of the vapour produced exceed,* that to which the liquid is exposed, it follows that a mix ture of liquids which have no tendency to dissolve one another will boil at a temperature below the boiling-point of either of them ; but when the liquids have an affinity for each other the boiling-point of the mixture will be above that of the more volatile constituent. The method employed by Gay Lussac for the measure ment of the pressure of aqueous vapour at low temperatures has not since been improved upon. He employed a barometer tube whose length was considerably greater than the height of the barometer, and having bent the upper portion (above the mercury) over so as to slope downwards at an angle of about 60 with the horizon, lie immersed the closed end in a culd mixture at the tempera ture for which the pressure was to be measured, and injected a little water into the barometer tube. The vapour pro duced condensed in the cold part of the tube, and this process of distillation continued until the whole of the water had evaporated from the surface of the mercury, leaving it free to rise and fall in the tube. The pressure of the vapour was afterwards always that due to the temperature of the coldest part of the tube, for if at any time it exceeded this pressure, condensation would com mence and continue until the pressure was reduced to this amount. A barometer tube dipping into the same trough of mercury, and containing no water, was placed by the side of the experimental tube, and the difference in the level of the mercury in the tubes was read by means of a microscope sliding on a graduated pillar, this difference obviously indicating the pressure of the vapour. The rate at which evaporation takes place has been the subject of much inquiry. In 1772 Dr Dobson of Liver pool (Phil. Trans., Ixvii.) placed a cylindrical vessel, 12 inches in diameter, by the side of a rain-gauge, and, allowing for the rain which fell into it, determined the total eva poration during each month for four years. Dalton and Hoyle imitated more closely the conditions presented by the soil, and filled a vessel three feet in depth with gravel and sand, covering it with earth and sinking it in the ground; a pipe was placed near the top and one near the bottom in order to collect any water which might be free to run off, while the amount of rain received was measured by a rain-gauge placed close to the vessel. At the commence ment of the series of observations the contents of the vessel were saturated with water, and the difference between the amount of rain received and of water that escaped by the pipes indicated the amount of evaporation. From observations of the rate of evaporation of water contained in a shallow tin dish Dalton concluded that at different temperatures in calm air the rate of evaporation is proportional to the maximum pressure of steam at that temperature, diminished by the pressure of the vapour already existing in the air, which pressure is determined from an observation of the dew-point, and that when the air is in motion the rate of evaporation increases with the velocity of the wind. It really depends not only on the temperature, but on the rate at which the vapour can escape from the neighbourhood of the liquid, and evapora tion therefore proceeds more quickly when the pressure of the air is diminished. Some considerations on the subject will be found in the article DIFFUSION. Many of Dalton s experiments were subsequently repeated in a modified form by Daniell, who examined the pressure of steam at various temperatures, and in the presence of other gases, as well as the rate of evaporation. The chief monument of DanielPs work on this subject is his dew- point instrument. Hutton was the first to suggest the determination of the hygrometric state of the air from the cold produced by evaporation; and Sir John Leslie employed the same method, in connexion with the differen tial thermometer. For the theory of Mason s dry and wet bulb thermometers, or, as it is sometimes called, August s psychrometer, see article DIFFUSION. In 1823 the determination of the maximum pressure of aqueous vapour at different temperatures was referred to a commission of the Academy of Paris, and the work was undertaken by Dulong and Arago. They measured the pressure of steam at temperatures ranging from 100 C. to 224 C., by observing the compression of a quantity of air imprisoned by mercury in a tube. About the same time a committee of the Franklin Institute of Pennsylvania : measured the temperature of steam in contact with water, at pressures varying from one to ten atmospheres; but the results of the two series of experiments did not agree very well. It was partly on this account that Regnault determined to investigate the subject more thoroughly, anc it is to him we are indebted for a table of the pressure o: aqueous vapour over a range of temperature varying fron 32 C. to 230 C. Some of his results, together witl i