306 MR. E. H. GRIFFITHS ON THE LATENT 
(after comparison with the barometer) the pressure of the vapour in the condenser. 
The tube H branched into two; one was connected with a water pump, by which 
the pressure could be brought down to about 20 millims., and the other with a 
GEISSLER’S mercury pump. 
Description of an Experiment. 
The dropper was filled by alternate boiling and cooling,* and was allowed to stand 
in a vessel of warm water until the temperature had fallen to about 5° above that 
to which it was to be exposed during the experiment. After removal from the water 
it was thoroughly dried externally and placed in a short glass tube closed at both 
ends by rubber corks, a precisely similar tube closed in the same manner being used 
as atare. The case and dropper when full weighed about 20 grams. 
The dropper was always filled some hours before an experiment and placed in the 
balance case until wanted. After the tank temperature had become steady the 
dropper and its case, having been weighed, were placed within a larger tube 
immersed in the tank water. 
The connections of the electric circuit having been completed, the calorimeter 
temperature (@,) was made coincident with the tank temperature (8), the current 
being then switched on to the alternative coil, and thus (as previously explained) 
the temperature of the external resistances was kept steady, even when the current 
was not passing through the calorimeter. The dropper and case were then removed 
from the tank, a silk thread was passed through a platinum loop, fused into the top 
of the dropper, and it was lowered into its place at the bottom of the tube h 
(Plate 5, fig. 1). The thread was withdrawn and an air-tight piston, consisting of a 
section of a rubber cork mounted on a glass rod, was thrust down / until it arrived 
at the top of the dropper. A slightly larger conical rubber cork, mounted on the 
same rod, closed the top of the tube h, therefore all diffusion or evaporation up the 
connecting tube was prevented, and no difficulty was experienced in closing it in 
such a manner as to be absolutely air-tight. 
It was necessary to delay the commencement of the experiment until the dropper 
and contained water had assumed the temperature of the calorimeter (6;). This 
took some time, although the temperature of the former must (owing to the previous 
immersion in the tank) have been very nearly 6,. Observation of the thermometer- 
galvanometer gave the extent of the cooling caused by the introduction of the 
dropping tube. The current was then switched on until 0, again equalled 6), the 
galvanometer was observed, and the process repeated until no further cooling effect 
was visible. 
It appeared possible that slight evaporation through the fine opening of the dropper 
might during this interval be the cause of some cooling, and I was for a time very 
* T found that it was necessary to fix a small capillary tube within the dropper, otherwise the 
contained water refused to start boiling when the external pressure was removed. 
