OM I/V, P M=1, Now, from the figure, Q M = Ο Μ tan Q O M = k, say, k being some constant ; :. (p−þ。) v=constant. Thus if we diminish the observed pressure by a constant quantity o, the product of the difference and the volume is constant. The observed pressure p is therefore the sum of a con FIG. XXIV. B M stant pressure to and a pressure, which satisfies Boyle's law-i.e. the actual pressure is that due to the air obeying Boyle's law together with a constant pressure, that of the aqueous vapour saturating the space at the given temperature. On varying the temperature the same law will be found to hold, but the pressure to will be different for different temperatures; and if Dalton's law is true, the values of po for different temperatures will correspond exactly with those given in Regnault's table of saturation-pressures of aqueous vapour. In carrying out the experiment it is very important that the temperature should be constant, as the pressure of the vapour changes greatly with temperature. Time must in each case be given for the air to become saturated. Experiment.-Verify Dalton's law. HYGROMETRY. -- Pressure of Aqueous Vapour.-The determination of the amount of water contained in the atmosphere as vapour is a problem of great importance, especially to meteorology. There are several ways in which we may attempt to make the determination, and the result of the experiment may also be variously expressed. The quantity of water which can be contained in air at a given temperature is limited by the condition that the pressure of the vapour (considered independently of the pressure of the atmosphere containing it) cannot exceed a certain amount, which is definite for a definite temperature, and which for temperatures usually occurring, viz. between – 10° C. and +30° C., lies between 2 mm. of mercury and 31.5 mm. Dalton concluded, from experiments of his own, that this maximum pressure, which water vapour could exert when in the atmosphere, was the same as that which the vapour could exert if the air were removed, and indeed that the dry air and the vapour pressed the sides of the vessel containing them with a pressure entirely independent one of the other, the sum of the two being the resultant pressure of the damp air (see the pre vious experiment, § 41). This law of Dalton's has been shewn by Regnault to be true, within small limits of error, at different temperatures for saturated air, that is, for air which contains as much vapour as possible; and it is now In the first edition of this work the words 'pressure' and 'tension' were used, in accordance with custom, as synonymous. In this edition it is intended to use the term 'pressure' only in referring to aqueous vapour. a generally accepted principle, not only for the vapour of water and air, but for all gases and vapours which do not act chemically upon one another, and accordingly one of the most usual methods of expressing the state of the air with respect to the moisture it contains is to quote the pressure exerted by the moisture at the time of the observation. Let this be denoted by e; then by saying that the pressure of aqueous vapour in the atmosphere is e, we mean that if we enclose a quantity of the air without altering its pressure, we shall reduce its pressure by e, if we remove from it, by any means, the whole of its water without altering its volume. The quantity we have denoted by e is often called the pressure of aqueous vapour in the air. Relative Humidity.—From what has gone before, it will be understood that when the temperature of the air is known we can find by means of a table of pressures of water vapour in vacuo the maximum pressure which water vapour can exert in the atmosphere. This may be called the saturation pressure for that temperature. Let the temperature bet and the saturation pressure e, then if the actual pressure at the time be e, the so-called fraction of saturation will be and the percentage of saturation will be e This is known as the relative humidity. 100 e Dew Point. If we suppose a mass of moist air to be enclosed in a perfectly flexible envelope, which prevents its mixing with the surrounding air but exerts no additional pressure upon it, and suppose this enclosed air to be gradually diminished in temperature, a little consideration will shew that if both the dry air and vapour are subject to the same laws of contraction from diminution of temperature under constant pressure,1 the dry air and vapour will contract by the same fraction of their volume, but the pressure of each will be The condition here stated has been proved by the experiments of Regnault, Herwig, and others, to be very nearly fulfilled in the case of water vapour. always the same as it was originally, the sum of the two being always equal to the atmospheric pressure on the outside of the envelope. If, then, the pressure of aqueous vapour in the original air was e, we shall by continual cooling arrive at a temperature-let us call it r- -at which e is the saturation pressure; and if we cool the air below that we must get some of the moisture deposited as a cloud or as dew. This temperature is therefore known as the dew point. If we then determine the dew point to be τ, we can find e, the pressure of aqueous vapour in the air at the time, by looking out in the table of pressures e, the saturation pressure at r, and we have by the foregoing reasoning 47. The Chemical Method of Determining the Density of Aqueous Vapour in the Air. It is not easy to arrange experiments to determine directly, with sufficient accuracy, the diminution in pressure of a mass of air when all moisture shall have been abstracted without alteration of volume, but we may attack the problem indirectly. Let us suppose that we determine the weight in grammes of the moisture which is contained in a cubic metre of the air as we find it at the temperature t and with a barometric pressure H. Then this weight is properly called the actual density of the aqueous vapour in the air at the time, in grammes per cubic metre. Let this be denoted by d, and let us denote by & the specific gravity of the aqueous vapour referred to air at the same pressure e and the same temperature, and moreover let w be the density of air at o° C. and 760 mm. pressure expressed in grammes per cubic metre. Then the density of air at the pressure and temperature, also expressed in grammes per cubic metre, is equal to 760 (1+at) where a coefficient of expansion of gases per degree centigrade, and therefore Now w is known to be 1293 and a = *00366 ; e= 760(1+00366 t) d. (1) If, therefore, we know the value of 8 for the conditions of the air under experiment, we can calculate the tension of the vapour when we know its actual density. Now, for water vapour which is not near its point of saturation 8 is equal to 622 for all temperatures and pressures. It would be always constant and equal to '622 if the vapour followed the gaseous laws up to saturation pressure. That is however, not strictly the case, and yet Regnault has shewn by a series of experiments on saturated air that the for760(1+*00366 t)d suffices to give accurately the 1293 × 622 mula e = pressure when d is known, even for air which is saturated, or nearly so, with vapour. We have still to shew how to determine d. This can be done if we cause, by means of an aspirator, a known volume of air to pass over some substance which will entirely absorb from the air the moisture and nothing else, and determine the increase of weight thus produced. Such a substance is sulphuric acid with a specific gravity of 184. To facilitate the absorption, the sulphuric acid is allowed to soak into small fragments of pumice contained in a U-tube. The pumice should be first broken into fragments about the size of a pea, then treated with sulphuric acid and heated to redness, to decompose any chlorides, &c., which may be contained in it. The U-tubes may then be filled with the fragments, and the strong sulphuric acid poured on till the |