Mr. Daniell on Evaporation. 33 
that of complete dryness, I had no opportunity of judging whether 
the elasticity of the vapour, as it rose from the surface of the water, 
varied in any degree with the pressure of the air, or whether any 
part of the increase of evaporation were dependant upon such vari- 
ation. To determine this point, I placed the sulphuric acid in a 
glass, of the diameter of 2°8 inches, so that its surface was very 
little more than equal to that of the water. The vessels were 
placed, side by side, upon the plate of the air-pump, and covered 
with the receiver. The temperature of the water and air was 52°, 
and the height of the barometer 29°8. The following table shews 
the dew point, which was obtained, at intervals of half an hour, at 
different degrees of atmospheric pressure :— 
Barom. ~— Temp. of Water and Air Dew Point 
29 8 
se SL agebllee been lla pics: ay, 2 apa teed cg: 
~ ge nl they t all Sante © be gear aude tute 
Oo tine dbinga ei 86 yee pepe nga 
FLO mean i amie hari agate 7 nr arthaaneideert aterm barn 2 
2 ne aga atte et 2 deter deter gamba 
2A peta et atts ent aint, ylde ages pong 
The differences of these results are so extremely small, and are 
moreover so little connected with the variations of density, that 
there can be no difficulty in regarding them as errors of observa- 
tion, and we may conclude, that the elasticity of vapour, given off 
by water of the same temperature, is not influenced by differences of 
atmospheric pressure. The equal surfaces of sulphuric acid and 
water here made use of, maintained, at the temperature of 52°, a 
degree of saturation equal to 570. I repeated the experiment, at 
the temperature of 61°, and the following are the results:— 
Barom. Temp. of Water and Air Dew Point 
29-6 
12 ie) a a atl A ia a RS RS 
Prgirivegay sow) © gipile, aagdgrgmaly: 96g 
Gay Dank ey sien Barris Meals V6 gD 
pepe el 30st) OPT yep gag 
POA OLE HBO IN WA 4S Tike 
PION ATS Var EB yt as 
