o4 Mr. Daniell on Evaporation. 
Under these circumstances, the amount of saturation was 651; 
an increase evidently dependant upon the force of the vapour, but 
not in exact proportion to its augmentation. 
Experiment 5. 
Being now desirous of ascertaining in what degree the tempera- 
ture of an evaporating surface would be influenced by differences 
in the density of the air, I made the following disposition of the 
apparatus:—To a brass wire, sliding through a collar of leathers, 
in a ground brass plate, I attached a very delicate mercurial ther- 
mometer; this was fixed, air-tight, upon the top of a large glass 
receiver, which covered a surface of sulphuric acid of nearly equal 
dimensions with its base. Upon a tripod of glass, standing in the 
acid, was placed a vessel containing a little water, into which the 
thermometer could be dipped and withdrawn by means of the slid- 
ing wire. The bulb of the thermometer was covered with filtering- 
paper. At the commencement of the experiment, the barometer 
was at 30:2 inches, and the temperature of the air 50°. Upon 
withdrawing the thermometer from the water, it began to fall very 
rapidly, and in a few minutes reached its maximum of depression. 
The following table presents the results of the experiment, for 
different degrees of the air’s density; the intervals were each of 
twenty minutes :— 
Barom. Temp. of Air Temp. of wet Ther. Difference 
1 a alt al dat tee: aia a ee al 
LT ee ee, ae Ca ae 
PA ee, FEOP (PO ER Re BRO AEG 
Se a OS eee SO Aa See 
1G 82? OY SR eo (See 
oh: si: = Sola le Mian a ae ok yrs 
Se RI Oe ot SSR ote 
Here, in an atmosphere which a former experiment has proved 
to be in a state of almost perfect dryness, we find that, at the full 
atmospheric pressure, the wet surface of the thermometer was re- 
duced 9°, It is worthy of remark, also, how small a quantity of 
water is required to produce this effect. It has been previously 
