HEAT OF EVAPORATION OF WATER. 311 
as shown by Col. III., absolutely unchanged. It is noticeable that its result 
(Table XI. (b)) is almost exactly the mean value of Experiments VI. to XII. 
I have included in these tables every experiment performed by the exhaust method 
with the exception of one, during a portion of which the chronograph ceased to mark, 
or rather marked continuously. Both accident and its cause were only discovered at 
the end of the experiment, when it was found that a loose piece of wire had short- 
circuited the chronograph circuit. 
32 
32> 
WEEE 
Notation used in Tables XI. to XIII. 
Number of experiment and date. 
Temperature of the experiment (4) on the nitrogen scale. 
m, the initial mass of dropper and contents, m, the final mass ; hence 
Mm, — Mz = mass evaporated = M. 
(M is in all cases the weight corrected to vacuo.) 
Time (in seconds) during which the electric current was passing through 
the calorimeter = t,. 
Time ¢, (in seconds) during which the stirring supply of heat was main- 
tained. ¢, = duration of experiment. 
Number of revolutions per second of the stirrer = 7. 
Ditference (6°) — @”)) between the initial (@)) and final (6”,) temperature 
of the surrounding walls. This is expressed in the nitrogen scale, the 
value of each millimetre of thermometer II. having been previously 
determined by the comparison referred to in Section IV. 
Let 6’, (initial calorimeter temperature) exceed 0’, by d’, and let 6”, (final 
calorimeter temperature) exceed 0”, by d”. 
Then this column gives value of d’ — d” deduced from the galvanometer 
swings by Table V., Section VI. 
Gives the capacity for heat of calorimeter and contents (C,,) at the tem- 
perature of the calorimeter 6, (‘Table IIL). 
The temperature of the Clark cells during the experiment. 
The value of R, at temperature 0, from Table VIL., Section VIII. 
The average pressure (p”) in the condenser during the experiment (in 
millimetres of Hg). 
The approximate pressure of saturated vapour (p’) at the temperature 0, 
(from ReGNAvLt’s tables). 
The difference between Cols. XIII and XII. This indicates the limit of 
fall of pressure from the flask to the condenser, 7.e., along about 
19 feet of narrow tubing. It must be remembered that owing to the 
presence of the H,SO, the pressure in the condenser fell off greatly, 
and Col. XIII. is only useful as indicating a value considerably 
exceeding the real difference between p’ and p’”. No use is made of 
this quantity in the reduction of the observations. 
