594 



H Y G R O M E T R Y. 



Hjrgrome- that water is maintained in the atmosphere, in a va- 

 U J- porous state, hjr the influence of heat alone ; and that 



x """' "Y 1 ^ it is supported in the air, not in consequence of any 

 chemical affinity for that fluid, but by its superior levi- 

 ty, its weight having'been shewn ( 34) to be to that 

 of air, under the same temperature and pressure, as 5 

 to 8. This important fact was first stated in a philo- 

 sophical manner by De I.uc, who considered vapour 

 to be an aeriform fluid, rendered elastic by the action 

 of caloric, independent of the presence of air. The 

 density which vapours can acquire, says he, (Idees stir 

 la Metenrologie, torn. i. p. 2G), or the degree of proxi- 

 mity which their particles can attain, without being 

 precipitated, has a maximum which is determinate in 

 the same temperature, but which changes with the 

 temperature, and increases as it is more elevated. Be- 

 yond this maximum of density, vapours, he adds, are 

 partially precipitated, till they are reduced within its 

 limits. These views were afterwards more fully de- 

 veloped in a memoir which he published in the Phi- 

 losophical Transactions for 1792. By his excellent ex- 

 periments on the force of vapour at different tempera- 

 tures, Mr Dalton gave to this theory a precision and a 

 generality which it by no means possessed before he 

 turned his attention to the subject ; though, it must be 

 admitted, that the opposite opinion, which ascribed the 

 formation of vapour to the influence of chemical ac- 

 tion, aided by temperature, could not be said to have 

 been fully disproved till Gay Lussac took a part in the 

 investigation, and demonstrated by a series of accurate 

 experiments, which Biot * afterwards illustrated by the 

 most rigid analysis, that vapour has the same mechani- 

 cal properties, whether it exists in a pure unmixed 

 state, or in combination with atmospheric air. As al- 

 most all the conclusions which we have deduced re- 

 specting the hygrometric state of the air are either 

 founded upon this fact, or intimately connected with it, 

 we were anxious not to quit the subject till we had 

 ascertained, by direct experiment, how far the quan- 

 tity of moisture in a given volume of air coincided 

 with the results laid down in rome of the preceding 

 Tables ; but the minuteness of the quantity of mois- 

 ture maintained in a vaporous state, even in the largest 

 receiver, together with the difficulty of abstracting the 

 whole of it from its mechanical admixture with air, 

 presented considerable obstacles to an accurate solution 

 of this nice and delicate problem. The method em- 

 ployed by Saussure, mentioned in 74, of introducing 

 small bits of moistened linen under a receiver, con- 

 taining a certain volume of dry air, is liable to many 

 obvious objections ; and after trying it on a large scale, 

 without obtaining results sufficiently satisfactory, we 

 abandoned it for the following mode of research, which, 

 from the coincidence of the results it afforded in the 

 same circumstances, is, we conceive, well calculated 

 to determine, with the utmost precision, the object in 

 view. The method alluded to, consisted in causing a 

 large volume of air, saturated with moisture, to pass 

 slowly in a small stream through a sufficient quantity 

 of sulphuric acid, or dry muriate of lime, cut off from 

 all communication with the atmosphere ; and then ob- 

 serving the increase of weight which these substances 

 acquired, in consequence of the air being transmitted 

 1 inscription through them. After briefly describing the apparatus 

 01' the ap- we employed, we shall state the results which we ob- 

 ~ tainetl in several experiments performed with the utmost 



care, at temperatures considerably' remote from each Hygnane. 

 other. ABCD Fig. 4. represents the gasometer, or tr y- 

 air-vessel used in these experiments ; its capacity was 

 11,320 cubic inches, and to preserve the air which it CCCXXVI 

 contained at an uniform temperature, it was enclosed pj n _ 4. 

 in another vessel filled with water, the temperature of 

 which was easily regulated. The air was expelled from 

 the gasometer by pouring water of the same tempera, 

 ture into the funnel RS, the stop-cock E being previ. 

 ously turned; it thus passed along the bent tube EFG, 

 one extremity of which G descended about |th of an 

 inch below the surface of a quantity of mercury in the 

 bottom of the vessel FVTL. The bent syphon tube 

 HIK contained a small portion of mercury, and served 

 to indicate the pressure to which the air was subjected 

 during the experiment. From the vessel FVTL, the 

 air was conveyed by the tube LMN into the vessel 

 MN, which was filled with concentrated sulphuric acid 

 to the depth of four inches. Passing through a very 

 small aperture at N, it ascended in a succession of bub- 

 bles through the acid, and at last made its escape at the 

 extremity of the tube OP, which was inserted under 

 the surface of mercury. The junctures at F, H, L and 

 O having been luted so as to be air-tight, it is easy 

 to see, that since all communication was thus cut off 

 between the external air and the acid in MN, any ad- 

 dition of weight which the acid acquired must have 

 been derived solely from the vapour in mixture with 

 the air, which was urged through it from the gaso> 

 meter. The acid, therefore, being carefully weighed 

 before and after the experiment, with an excellent ba- 

 lance, which, when loaded with a pound, turned witli 

 the 500th part of a grain, the increase of weight which 

 it received afforded an accurate test of the weight of 

 the vapour which existed in mixture or combination 

 with the air. To prevent any of the moisture from 

 being deposited in the tubes, the temperature of the 

 air on which the experiment was made, was kept by 

 means of the water in the external vessel VVVYX two 

 or three degrees below the temperature of the sur- 

 rounding air; and as the direct communication between 

 the moist air in the gasometer, and the acid which ab- 

 sorbed the vapour, was cut off by the mercury in the 

 bottom of the vessel FVTL, the quantity of moisture 

 dissolved could not be affected by the diminished elas- 

 ticity of the air arising from the incessant absorption 

 of the vapour. Some of the experiments were repeat- 

 ed with muriate of lime instead of sulphuric acid, by 

 diffusing a solution of it over an extended surface, 

 by means of flannel, which was afterwards well dried, 

 and then wrapped loosely round the tube, which de- 

 scended to the bottom of the vessel MN. With this ap- 

 paratus we obtained the following results : 



* In alluding once more to the hygrometric researches of this distinguished mathematician and philosopher, we gladly embrace the 

 opportunity of correcting a slight mistake which we committed, in giving an account (see 77. of this article) of the labours of Gay 

 l.ussac to determine the relation between the tension of vapcur, fcnd the indications of Saussure's hygrometer. We have found, 



