relating to Hygrometers and Evaporation, Ijl %^ 



meter indicated 5S°.5 Fahr. and the moist 40°.4, giving a de- 

 pression of 13°.l. The height of the barometer was nearly 

 30 inches ; but exactness in this, as we shall shortly see, was of 

 no moment. 



There is, however, reason to think, that a quantity of air, con- 

 fined in a vessel along with a drying substance, such as sulphu- 

 ric acid, can never be rendered perfectly dry, if it, at same time, 

 contain the bulb of a thermometer covered with wet linen ; be- 

 cause this, to a certain extent, will continually supply it with 

 moisture, which must require some time to pass to the acid and 

 be there absorbed, and the more so as the vessel is larger *. 



To try the effects of a smaller vessel, I put a little sulphuric 

 acid into a small spheroidal flask having about the 27th of the 

 capacity of the former ; and introducing a single thermometer 

 with its ball moistened as before, I fastened Its stem in the neck 

 of the flask. To note the temperature of the included air, I 

 kept the flask and another thermometer immersed in a jar of 

 water, which was frequently stirred. The flask was often turn- 

 ed on its side, rolling it round to keep the interior surface wetted 

 with acid. At the end of about fths of an hour, the full effect 

 seemed to have been attained. The thermometer, in the water, 

 stood, as from the beginning, at 53" Fahr. and that within the 

 flask at 39°.9, giving a depression of 13°.l as before. This and 

 the first experiment were repeated some days after, with the same 

 result -f*. 



From these experiments I was almost led to the conclusion, 

 that if the interior surface of a bottle be kept wet with acid, its 

 size should be of no consequence. But reflecting, that the balls of 

 the thermometers, in the larger bottle, had been kept in motion, 

 and that within an inch of the acid in the bottom, I was induced 

 to try what effect it would have to fasten the moist ball as near- 

 ly as possible in the centre of the larger bottle. Upon doing so, 



* Various liquids are known to produce greater cold in the surface from 

 which they evaporate than water does. Yet it is curious, that so volatile 

 a fluid as oil of turpentine should have no effect in this way ; and a covered 

 thermometer, first dipped in oil of turpentine, and then in water, undergoes 

 the same depression as if no turpentine were present. 



t The like coincidence I find, obtains at 80" Fahr., the depression then 

 amounts to 24.°3. 



