HEAT OF EVAPORATION OF WATER. 337 
In Paper A. I have given a full account of the determination of the ‘ water 
equivalent ” (W,) of this calorimeter, and I have every confidence in the values there 
given. 
From Table VI. of that Paper I extract the following :— 
6, W). 
20 80°11 
30 80°90 
40 82°19 
50 83°39 
During these experiments, the silver flask contained a mercury thermometer (G,) 
which was used, during my earlier L experiments, to indicate the internal temperature 
of the flask. As described (ante), a metal conical tube had been cast on to the stem 
of this thermometer by means of the alloy, and the tube carefully ground into the 
neck of the flask to prevent any diffusion of vapour up the glass tube, down which 
the thermometer passed. After I adopted the “ exhaust” method of performing the 
L experiments, this thermometer was removed from the apparatus; this occurred 
between Experiments II]. and IV., Table XXIV. I had forgotten the circumstance, and 
when I reduced the results, I was much troubled as to the different values of C,, given 
by Experiments II. and IV., which were at about the same temperature. It was some 
time before the cause of the discrepancy suggested itself to me. I then obtained an 
approximation to the capacity for heat of thermometer G, and tube, as follows: The 
temperature of both calorimeter and tank being steady, the bridge was adjusted until 
the galvanometer ceased to swing. Thermometer G, was then suspended above the 
tube communicating with the flask, and its temperature read by the reading micro- 
scope. It was then rapidly lowered into its place, and observed until its temperature 
became steady (this took place in about 4 to 5 minutes). The tank being at 40° and 
the external temperature about 20°, the thermometer rise was about 20°, and a small 
error in reading was of little consequence. The bridge contact-maker was then 
re-adjusted, and the change in temperature of the calorimeter deduced trom the 
difference in the readings. Hence the capacity for heat of G, could be found. Four 
experiments were performed with the following results :— 

(al) aise 
(2) 1°65 
(3) 1:62 
(4) 1°58 
Means 92. 159 

MDCCCXCV,—A. xs 
