400 Experimental Researches on the Depressions [July, 



Dalton himself, as regards the quantity of water evaporated. Theo- 

 retically, however, it has no influence on its temperature ; and this is 

 confirmed by experiment, under certain limitations. 



With such an appalling complication of influences to be traced out, 

 l t is hardly to be wondered at that M. Gay Lussac himself should have 

 given up the prosecution of the wet-bulb problem, or that the Editor of 

 the Royal Institution Journal* should have joined in its condemnation 

 at a time when the elegant method of Daniell was winning general fa- 

 vor. Nevertheless, independently of its direct preferability as the most 

 simple mode of registering the humidity of the air, the problem itself 

 is of the highest importance, in the solution not only of very many 

 phenomena in pneumatics and meteorology, but of such standard doc- 

 trinal points of theory, as the latent heat of gases and steam ; and 

 others of practical utility — as the artificial production of ice and cold. 

 I shall have occasion to adduce a few illustrations ere I conclude ; 

 but I must now proceed to my first series of experiments. 

 § 1 . On the curve of maximum depression. 



The apparatus used for drying the air is sectionally depicted in 

 Plate XXI. tig. 1, where a is a dish containing concentrated sulphuric 

 acid enclosed in a 1 20 pint gasometer. Another similar dish rests in 

 the glass double bell receiver b, wherein are suspended a hair hygrome- 

 ter (the only instrument applicable as a tell-tale, and indeed an inva- 

 luable hygrometer for every purpose) and a delicate thermometer. 

 Through this receiver the air of the gasometer passes to the stopcock 

 and short glass tube c, in which is placed a small thermometer, cover- 

 ed with muslin and dipped in distilled water at the moment before the 

 experiment commences. 



The only difference in the order of M. Gay Lussac's experiments, 

 being, as I have stated above, that he employed chloride of lime with- 

 out a tell-tale hygrometer, while in my first Benares series I employed 

 the same salt with this addition, it would be easy to apply to that 

 philosopher's results the correction I found necessary for the want of 

 complete desiccation in my case. At all events, as his series compre- 

 hends low temperatures, which were beyond my reach in India, it will 

 render my review of the question more complete to insert his valuable 

 table, converting the centigrade expressions into those of Farenheit's 

 thermometer. In the fourth column I have added the aqueous ten- 

 sions t at the wet-bulb temperature ; and in the fifth, the quotients of 



* Jour. Roy. lust. XV. 296. 



t By Biot's formula founded on Dalton' s experiments, and published in 

 the Edinburgh Encyclopedia, Mr. Anderson's article Hygrometry. 



