308 Experimental Inquiries respecting Heat and J^apor. 



the temperature to reach SOO"^, I applied equal portions of water to 

 each cup, and found their actions precisely alike. 1 then placed and 

 spread, as lightly as possible, a minute portion of olive oil, forming 

 a thin film over the surface of one of the cups, allowing the other to 

 remain clean. On renewing the applications of water, it was found 

 that the oiled took four times as long as the clean surface to vaporize 

 a certain quantity of water. On elevating the temperature, the oil 

 itself was gradually evaporated, and the water found occasional ad- 

 mittance to the surface. Hence the difference was gradually dimin- 

 ished, and the wonted action of the iron restored, but the addition 

 of fresh portions of oil, again reduced temporarily the vaporization 

 on the surface to which it was applied. But as the temperature was 

 more elevated than before, the oil likewise became sooner dissipated. 



By exposing the bar in a similar manner, and ascertaining that 

 two contiguous cups, equally remote from the centre of flame, were, 

 when both clean, precisely alike in regard to the rapidity of evapo- 

 ration at a high temperature, I lubricated one with plumbago, laid on 

 by rubbing a piece of that substance over the interior, without how- 

 ever leaving any dust or small bits of the mineral to serve as nuclei 

 for the water to seize upon. The other cup was left clean as before. 

 Equal portions of water at 60° were now laid simultaneously upon 

 the bottom of the two cups. The mean result, of six experiments 

 in each, was that the cup with plumbago required eighty four seconds 

 to evaporate its liquid, while the cup without plumbago took but forty 

 one for that purpose. The portions of liquid used were single drops 

 for the respective experiments. 



To ascertain the effect of thickening the water into a thin paste, I 

 put a large tea-spoon full of flour into an ounce of water, and laid 

 one-fourth of an ounce of the mixture on the bottom of the iron ba- 

 sin, kept red hot over the fire. The evaporation took place, and 

 the paste became dry in seventy-eight seconds. Under precisely the 

 same circumstances, clear water, of the same temperature as that 

 mixed with the flour, required one hundred and thirty-eight seconds 

 to evaporate one-fourth of an ounce. 



The action on clear water was rendered much more rapid, howev- 

 er, by covering the surface with a circle of white paper laid on imme- 

 diately after the water was put into the basin. The evaporation then 

 took place in seventy-two seconds. In another experiment, in which 

 the circle of paper was smaller than that of water, the time was in- 

 creased to ninety seconds. In both of these cases, the acceleration 



