DETERMINATION OF THE SPECIFIC HEAT OF WATER. 423 
by filter-paper, was suspended from three platinum wires attached to a copper 
terminal. The insulation of the voltameter was amply sufficient, and the contacts 
were all sufficiently good to allow a perfectly steady current to pass. In order 
always to bring back the dish to the same condition, the silver deposit of the previous 
experiment was always dissolved away before a new deposit was made. The dish 
was washed successively with (1) concentrated nitric acid, (2) a solution of potash, 
(3) tap water, (4) distilled water. After drying in air-bath the empty dish was 
weighed two or three times with the usual precautions. The silver deposit was treated 
in the manner described by Lord RayuercH. Immediately after an experiment the 
silver solution was removed from the dish, the deposit washed three times, and left 
overnight with distilled water. After washing again next morning, the deposit was 
dried in an air-bath, first at 100° C., subsequently the temperature of the bath was 
raised for 10 or 15 minutes to 160°C. The dish was then again weighed two or 
three times. The distilled water used in washing left no residue on evaporation 
from platinum foil, nor gave any visible turbidity with silver nitrate solution. The 
voltameter was efficiently protected from dust, which might otherwise have got 
entangled in the silver. The deposit always firmly adhered to the dish in charac- 
teristic radial lines, and no difficulty was ever experienced in the washing. 
Correction of the Weight to Vacuo. 
In correcting the weighings to vacuo we must distinguish between the bowl and 
the silver. As regards the bowl, it is only the difference in the density of the air 
when the two weighings are taken which will affect the result. We found it, there- 
fore, most convenient to reduce the weighings to the normal state of the atmosphere 
instead of to vacuo. The correction k to vacuo may be written k = wpa, « being 
a constant and p = py /yto/hot,, where 
Po = density of air at 76 centims. and 15° C., 
h, = height of barometer at time of weighing, 
t, = absolute temperature of balance case, 
h, = 76 centims., 
ty = 288°. 
Representing the changes in barometric pressure and temperature by dh and dt 
respectively, we may express the difference in the buoyancy correction from that 
which would hold if the temperature was 15° C. and barometric height 76 centims. by 
dh = 0 4 dh — hy dt}. 
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