1826*] On the Quantity of Vapour in the Atmosphere, 8$c. 97 



Article III. 



Methods of experimentally determining the Quantity of Vapour in 

 the Atmosphere^ and the Specific Gravities of Gases mixed with 

 Vapour, By John Herapath, Esq. 



(To the Editors of the Annals of Philosophy.) 



GENTLEMEN, Cranfordy July 18, 1826. 



In the Annals of Philosophy for Nov. 1821, p, 375, &c. I have 

 briefly explained the theory of two methods of determining the 

 quantity of vapour at any time in the atmosphere. One of these 

 methods was invented by Mr. Dalton who, I have shown in the 

 •above cited pages, has, by some oversight, miscomputed the 

 experimental results. Aware that simple and easy methods of 

 ascertaining the relative and absolute quantities of vapour in the 

 air are in many researches of importance, especially as the sensibi- 

 lity and accuracy of the common hygrometers are known so to 

 vary as in a few years to become almost useless, I have thought 

 it advisable to detail a little more methodically than I have the 

 principles and application of the above methods, and to add a 

 third which, I beheve, is new. This last method hss the advan- 

 tage of combining greater accuracy with more extensive 

 utility. 



First, or Mr. Dalton's Method. 



Let the water in a clean thin glass vessel be gradually cooled, 

 either by pouring into it colder water, or by mixing with it nitre, 

 sal ammoniac, Sec. until the vapour of the air begins to be 

 deposited on the surface of the glass in the form of dew, which 

 is just when the glass begins to appear dull. Let the tempera* 

 ture of the water at this moment be noted ; and from Mr. Dalton's 

 experiments, or his theorem, or from that one which I have 

 given, Annals for Dec. 1821, p. 434, let there be taken the 

 corresponding tension of the vapour, and it will be equal to the 

 elastic force of the vapour in the air. For since the cold stratum 

 ftt the surface of the glass must have the same elastic force as 

 the air, and since, at the very commencement of the deposition, 

 the proportions of air and vapour in this stratum are the same as 

 in any other part of the air, the ratio of these elasticities must 

 be the same ; and consequently the elastic force of the vapour 

 in the stratum is the same as in other parts of the air. 



Mr. Dalton, in order to get the elastic force of the vapour in 

 the air, increases the before found tension in the ratio of 448 -f- 

 the Fahr. temperature of the water to 448 + the temperature of 

 the air, not considering that from the very principles of fluidity 

 every part of the air, whether in contact with a colder body or 

 not, must be pressed by the same force, and therefore have the 



ISiiw Series, vol. xii. h 



