. The hygrometers which have been pro posed on the principle of a change of weight arising from the absorption of moisture, are liable to still greater ob Changes of weight may indeed tfflasured with greater accuracy by the common or torsion balance: but in the present case they are so small, that the particies of dust which are at all times floating in the atmosphere may produce a great alteration in the results. A great variety of substances which attract moist ure have been employed, such as sponge, cotton, bibulous paper, caustic potash, the deliquescent salts, sulphuric acid, &c. • but the indications which they give are deserving of very little credit. Chan ges of property indicated by the torsion of cords formed of gut, hemp, cotton, &c., and the torsion of certain vegetable fibres, are still more fallacious.
2. Hygrometers on the Principle of Con densation.—The instruments of this class are of a far more refined nature than those which we have been describing. In order to give an idea of the general principle on which they depend, let us conceive a glass jar, having its sides per fectly clean and transparent, to be filled with water, and placed on a table in a room where the temperature is, for exam ple, the temperature of the water being the same as that of the room. Let us next suppose pieces of ice, or a freez ing mixture, to be thrown into the water, whereby the water is gradually cooled down to 55, 50, 45, &c., degrees. As the process of cooling goes on, there is a cer tain instant at which the jar loses its transparency, or becomes dim ; and, on attentively examining the phenomenon, it is found to be caused by a very fine dew or deposition of aqueous vapor on the external surface of the vessel. The precise temperature of the water, and, consequently, of the vessel, at the instant when this deposition begins to be form ed, is called the dew point, and is capable of being noted with great precision. Now this temperature is evidently that to which, if the air were cooled down, under the same pressure, it would be complete ly saturated with moisture, and ready to deposit dew on any body in the least de gree colder than itself. The difference, therefore, between the temperature of the air, and the temperature of the water in the vessel when the dew begins to be formed, will afford an indication of the dryness of the air, or of its remoteness from the state of complete saturation.
But the observation which has now been described is capable of affording far more interesting and precise results than a mere indication of the comparative dry ness or moisture of the atmosphere. With the help of tables of theeLwtio force of aqueous vapor at different Wm peratures, it gives the means of deter mining the absolute weight of the aque ous vapor diffused through any given volume of air, the proportion of vapor existing in that volume to the quantity that would be required to saturate it, and of measuring the force and amount of evaporation.
The elastic force of aqueous vapor at the boiling point of water is evidently equal to the pressure of the atmosphere. Thihmay be assumed as corresponding to a column of mercury 30 inches in height. Mr. Dalton, in the fifth volume of the Manchester Memoirs, has given the details of a most valuable and beautiful set of experiments, by which he ascer tained the elastic force of vapor from water at every degree between its freez in g and boiling points in terms of the column of mercury which it is capable of supporting. As the same experiments have since been frequently repeated, and the different results present all the ac cordance which can be expected in so delicate an investigation, the tension of vapor at the different temperatures may be regarded as sufficiently well determin ed. Supposing, then, we have a table exhibiting the elasticity or tension corre sponding to every degree of the thermo meter, the weight of a given volume of vapor, for example a cubic foot, may be determined as follows : Steam at 212°, and under a pressure of 30 inches of mercury, is 1700 times light er than an equal bulk of water at its greatest density, or a temperature of about 40°, and a cubic foot of water at that temperature weighs 437272 grains ; the weight, therefore, of a cubic foot of steam at that temperature and pressure is 437272÷170257.218 grains. Hence we may find the weight of an equal bulk of vapor of the same temperature under any other given pressure, suppose 0.56 of an inch ; for the density being directly as the pressure, we have 30 in.: 0•56 in. : : 257.218 grs.: 4.801 grs., which is the weight required.
Having found the weight of a cubic foot of vapor under a pressure of 0.56 of an inch, and at the temperature 212°, we may find its weight under the same pres sure at any other temperature, suppose 60°. It is ascertained by experiment that all aeriform bodies, whether vapors or gases, expand the 1-480th part of their volume for every accession of tempera ture equivalent to one degree of Fahren heit's scale ; therefore, reckoning a vol ume of gas at 82° as unity, its vol ume at 60° is to its volume at 212° as 1 + is to 1+1N; or as 1.058: 1.375; and the density and weight being in versely as the volume, we have 1.038: 1.875 : : 4.801 grs. 6.222 grs.