HEAT OF EVAPORATION OF WATER. 309 
thermal and electrical balances had been finally adjusted there were often intervals of 
more than ten minutes when no alterations had to be made. 
From beginning to end, the task of Observer II. was that of announcing the 
galvanometer swings—a monotonous and uninteresting operation, which, however, 
required constant attention. 
When the experiment was approaching its termination a close watch had to be kept, 
for, unless the current was switched off the instant a sudden rise showed that all the 
water had been evaporated, the lapse of a few seconds would have raised 6, consider- 
ably above 6. If, on the other hand, the current was turned off too soon, it was 
always possible to switch it on again and raise @, up to @. After the first 
experiment it was easy to calculate (knowing the weight of the empty and the full 
dropper) the approximate time of ending. As a rule I cut off the current two or three 
seconds before that time, then increased the vacuum considerably to make sure 
of evaporating off the last drop of water, again established the current, and brought 
6, to just below 6, repeating this process as often as necessary——all the actions 
recording themselves on the chronograph tape. When @, had become absolutely 
steady, or only showed the slight increase due to the stirring, it was safe to assume 
that all the water had been evaporated. 
The tap T, was then finally closed, the tap T, slowly opened, and the air from the 
drying bottles S and P allowed to pass in through the 30-feet coil, C,, in the tank. The 
increase of 0, caused by the compression of this air was found, by observation, to be 
the same as the depression (previously referred to) which took place during exhaustion; 
and it was to allow for this rise that @, was set slightly below @)._ When the internal 
_ and external pressures had become equal, T, was closed and the current again switched 
on if necessary, until the swing became the same as the initial swings. If, however, 0, 
exceeded @,, this was not possible, and a correction had afterwards to be made for any 
difference. 
After repeated observation had shown that @, was steady, Observer II. read, as 
before, three galvanometer swings, pressing his chronograph key at the middle one. 
This record gave the termination of the interval ¢,, the time during which the stirring 
heat had to be estimated. 
It will be seen from the preceding account that ¢, always considerably exceeded ¢,, 
the time of electrical supply. 
The plugs closing the tube h h’ (Plate 5, fig. 1) were now withdrawn, a wire ending 
in a hook passed down the tube, the dropper extracted by means of its platinum loop, 
and immediately returned to its case, which was at once corked and placed on the 
balance, and afterwards weighed, with the various precautions previously referred to. 
The only remaining operation was that of translating the chronograph tape which 
gave the value to 3/5 second of ¢,, t,, and the time of each thousand revolutions of 
the stirrer. 
Remarks on Tables XI. to XIII. 
Tables XI., XII., and XIII. give the experimental results; Table XI. those at 
