262 PHILOSOPHICAL TRANSACTIONS. [aNNO 17Q2. 



on even the acquisitions of weight being proportional to the increase of moisture 

 in the medium ; since they do not keep the same pace in different substances. 



The other means was, to introduce in a dry vessel successive equal quantities 

 of water without opening the vessel, and to observe their effect on the hygroscope. 

 He made, last year, a first attempt of that experiment, which succeeded in re- 

 spect of the introduction of water in a space of a known small degree of moisture ; 

 but the event confirmed also the uncertainty that he suspected in that method, 

 because of a variable share of water retained by the vessel itself. 



Having now summed up the series of propositions which connect together in 

 one system the whole of the fundamental phenomena of hygrology and hygro- 

 metry ; the only part of that system which remained to be proved by immediate 

 experiments is, that link between the 2 classes of phenomena, namely ; " That 

 in vacuo, as in air, the product of evaporation affects the hygroscope as it does 

 the manometer." That experiment, he says, is now made with a sufficient de- 

 gree of regularity ; and the more so, as it has been executed by Mr. Haas, in 

 one of his air-pumps, with some of the whale-bone hygrometers, made by him- 

 self : and Mr. D. now gives its result, titled, 



Experiments on evaporation, in air and in vacuo. — After mentioning some 

 preliminary principles, he adds : Such is the general law of steam, as it results 

 clearly from the whole of the experiments; but in particular cases, it is subject to 

 anomalies from various causes, among which are the following. If the water that 

 evaporates be warmer than the space which receives the steam, more moisture is 

 produced in that space, or the quantity of steam is greater in it than by an equal 

 temperature in both ; and vice versa. More or less distance of the part of the 

 space where the hygrometer stands from the sides of the vessel, produces also 

 anomalies ; as according to their own state of moisture, if near enough, they 

 have an influence on moisture in that space ; and this is often the case in some 

 measure when the vessels are too small. Lastly, differences in the temperature 

 of the whole or of some part of the vessel, comparatively with that of the space, 

 are the most common causes of anomalies ; for steam is alternately decomposed 

 and reproduced by those differences, and when they have once begun in a vessel, 

 there is no certain means to bring it to a regular course of phenomena, except by 

 beginning again, or by a long equal temperature. He comes now to the experi- 

 ments, in which he indicates some effects of those causes ; and then concludes : 



In comparing the results of these experiments, moisture is generally greater in 

 proportion to the temperature. But, setting this aside, and comparing the mo- 

 tions of the hygrometer and the thermometer, it is evident that they are inde- 

 pendent of the modifications of air; and that it may safely be concluded : " That 

 the product of evaporation is always of the same nature, namely, an expansible 



