HEAT OF EVAPORATION OF WATER. 307 
uneasy about this point. I found, however, that if the tube, after filling and weighing, 
was placed in a desiccator for 12 hours with the opening uncovered, the loss of weight 
was less than 0°005 gram; it was, therefore, not probable that any appreciable 
evaporation took place during the interval occupied by the temperature adjustment. 
Again, the dropper, after being filled, was removed from the beaker at a higher tem- 
perature than that of the tank, so that immersion in the calorimeter would not lead 
to the expulsion of any water, provided that the small air-bubble formed at the. 
lower end when the dropper was cooled in the balance case did not rise into the 
upper part of the instrument, in which case the re-heating would possibly cause a 
small expulsion, owing to the warming of the air-bubble, which was previously absent 
from the tube. I think it just possible that this happened in some of the earlier 
experiments (Nos. I. to V.), but, after No. V., I adopted a new form of tube, of which 
the lower end was first turned up for about 1 centim., and then bent at right angles. 
Thus any air-bubble formed during the cooling remained at the open end, and also the 
water, when ultimately expelled, was thrown against the side of the flask (see Plate 5, 
fig. 1). 
The results of experiments performed after this alteration are in closer agreement 
than preceding ones. 
When @, was found by observation to have become quite steady, the contact-maker 
of the bridge was carefully adjusted to the bridge null-point (see p. 288), and the 
swing (if any) of the galvanometer was read. If 6, was found to be lower than 6), a 
further adjustment was made. In cases where 6, slightly exceeded 6), no adjustment 
was possible without again removing the dropper, so, unless the difference was 
considerable (¢.g., a swing exceeding 50 or 60, that is, a difference of about 0°01 C., 
see Table V., supra), the experiment was proceeded with. Three observations were 
taken of the swing, and the chronograph tape was marked during the second observa- 
tion. This gave the commencement of the interval of time ¢,, 7.e., the time during 
which the stirring supply had to be estimated. 
The pressure in the condenser B had been previously brought down below that 
required during the experiment, and, immediately after the observer at the gal- 
vanometer had registered the swings, the tap T, was opened. The manometer M at 
once showed an increase of pressure, due to the air in the flask and tubes expanding 
into the condenser. The expansion of this air produced a visible cooling effect, 
causing a galvanometer swing of about 90—equivalent to nearly 0°02 C. (Table V., 
supra). The manner in which this loss of heat was compensated for will be described 
later. 
The water pump having been cut off by the tap Ty, the mercury pump was if 
necessary brought into action. ‘The moment at which the discharge from the dropper 
commenced was indicated with great accuracy by the galvanometer and announced 
by Observer No. II. The electric current was at once switched on to the calorimeter 
circuit (the action recording itself on the chronograph tape) and the electric balance, 
2R 2 
