HEAT OF EVAPORATION OF WATER. 305 
A tube of this kind (which I shall call a ‘ dropper”) is shown in place in Plate 5, 
fig. 1. It was filled with water in the same manner as a “ weight thermometer.” 
To ascertain its action, I placed it within a wide glass tube whose lower end was 
closed. A constriction in the wide tube enabled me to stand the “dropper” within 
it, so that the narrow portion projected downwards (without touching the bottom) in 
the same manner as it would do when 7m situ above the flask. The upper end of the 
enclosing tube was connected with the exhaust pumps, a clamp being fixed on the 
connections. I found that when the pressure fell below that of the aqueous vapour, 
the water in the dropper was discharged into the surrounding chamber in a succession 
of small drops, but that if the communication with the exhaust was closed the 
dropping ceased. If the vacuum was maintained, the drops first thrown on the walls 
disappeared while a fresh supply was ejected. I concluded that when the space was 
absolutely saturated there was equilibrium, and I found that the disturbance of that 
equilibrium, caused by an almost imperceptible decrease in the pressure, was sufficient 
to maintain the flow. The size of the orifice of the dropper did not appear to be of 
any consequence, as I ascertained by experiment. Of course a certain amount of 
water-vapour must have been formed to saturate the space left within the dropping 
tube as the water retreated. It is evident, however, that the quantity thus evaporated 
would be very small. At the highest temperature at which I have yet worked (50° C.), 
the specific volume of water-vapour is somewhat below 12,000, and as the volume of 
the droppers used did not exceed 4°1 cub, centims., the weight of water required to 
saturate this space would be about ‘001 gram, and at lower temperatures much less. 
The greater portion of the heat required for such evaporation must, however, have 
been taken from the calorimeter, for the shoulder of the dropper rested on the metal 
ring at the base of the tube hh’ (Plate 5, fig. 1), and this ring formed a portion of the 
calorimeter. In order to make certain of this matter, I lowered the filled dropper 
into place, the contained water being slightly cooler than the calorimeter temperature, 
and deduced its water equivalent in the manner described in Appendix II., where I 
show how the capacity for heat of the thermometer (G,) was ascertained. The weight 
of water and glass in the dropper being known, its water-equivalent could also be 
alternatively obtained by calculation. It was difficult to accurately ascertain the 
temperature of the dropper just before lowering it into place, but the experimental 
results were in practical agreement with the calculated ones, and heat which dis- 
appeared within the dropper must therefore have been taken from the calorimeter. 
It is thus evident that no correction is rendered necessary by this internal 
evaporation. 
The adoption of the exhaust method involved certain changes in the exterior 
connections. The weighing bulbs and mercury trap were replaced by the bottle B 
(Plate 6, fig. 1). The connecting tubes which passed into this bottle were ground to 
fit and no corks were used. The calorimeter exit tube did not dip into the H,SO, 
but terminated about an inch above the surface of the acid. The manometer gave 
MDCCCXCV.—A. 2R 
