308 Experimental Inquiries respecting Heat and Vapor. 
the temperature to reach 300°, 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 a nucler 
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, | 
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 jron ba- 
ace, and 
the paste became dry in seventy-eight seconds. Under precise 
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, how 
er, by covering the surface with a circle of white pape? laid on imme- 
diately after the water was put into the basin. ‘The evaporation r. 
took place in seventy-two seconds. In another experiment, 1? whic 
the circle of paper was smaller than that of water, the time was 
creased to ninety seconds. In both of these cases, the accelerate” 
ev- 
