VIII
191
EVAPORATION OF LIQUIDS
In deter-
The
but of course it now took place under greater pressure than it
did at the same temperature before the air was introduced.¹
Three quantities, h, P, and t, had to be measured.
mining the first there was no difficulty. The determination
of P was not simply a case of measuring accurately the
difference of level; large corrections on account of the ex-
pansion of the mercury, etc. had to be applied, and some of
these were much larger than the quantity whose value was
sought. But by a careful application of theory and by special
experiments these corrections could be so far determined that
the final measurement could be relied upon to about 0.1 mm.
The outstanding error was so small that the greater part of
the observations would not be injuriously affected by it.
most uncertain element was the determination of t. I thought
it was safe to assume that the true mean temperature of the
surface could not differ by more than a few degrees from the
temperature indicated by the thermometer when the upper
end of its bulb (about 18 mm. long) was just level with the
surface; and it seemed probable that of the two the true
temperature would be the higher. For the bulk of the heat
was conveyed by the rapid convection currents; these seemed
first to rise upwards from the heated walls of the vessel, then to
pass along the surface, and finally, after cooling, down along the
thermometer tube. If this correctly describes the process, the
bulb of the thermometer was at the coolest place in the liquid.
With this apparatus I carried out a large number of experi-
ments at temperatures between 100° and 200°, and at nine
different pressures (i.e. with nine different admissions of air).
The separate observations naturally showed irregularities; but
unless some constant error was present, they undoubtedly point
to the following result:-The observed pressure P was always
smaller than the pressure P, of the saturated vapour corre-
sponding to the temperature t; at a given temperature the depth
of the layer which evaporated in unit time was proportional
to the difference P-P; when this difference was 1 mm. the
depth of the layer which evaporated per minute was 0.5 mm.
1
During the observations there was no air in the retort or the connecting
tube. Thus the introduction of the air does not invalidate the title of this
paper. The title, indeed, has only been used for brevity in place of a more
precise one.