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the calorimeter 1 constructed. A ring was etched near the 
top of the wide outer tube, which was then filled with water, 
and the india-rubber stopper pressed in till it was level with 
the ring (the excess of water escaping by the index tube) 
and the whole weighed. This was repeated several times, 
and it was found that (provided the temperature were con- 
stant) the weights differed only by two or three centi- 
grammes at the most. If, however, the stopper were per- 
manently fixed in and the index tube and thermometer 
and interior tube pushed into their proper marks afterwards 
the weights differed only by a few milligrams. It was thus 
seen that the volume of gas in the apparatus could be ad- 
justed with as great accuracy as by ground stoppers and 
with less chance of leakage. 
The apparatus consists then of a wide outer tube having 
a horizontal ring etched round it (in case it may be neces- 
sary to replace the stopper) and fitted with an india-rubber 
stopper through which the thermometer, reception tube and 
index are inserted. The reception tube may also have a 
ring etched round it, and be filled with water or other liquid 
to that mark, in order to cause any non-conducting body to 
give up its heat quickly to the glass, but I generally prefer 
to use the apparatus dry and stop the end of the reception 
tube with cork in order to prevent convection currents. 
When this latter method is used the apparatus is always 
ready for use, and requires no other preparation than placing 
the index in position, which is managed as follows : — With 
regard to the substance of the index I have found mercury 
not sufficiently mobile, so I have used instead a coloured 
aqueous solution preferably containing a little glycerine. 
The calorimeter is firmly clamped and enclosed in wool, and 
a small drop of coloured liquid is made to touch the end of 
the index tube, when it is at once drawn in. The number 
on the scale of the thermometer leve] with the stopper is 
read off, and the thermometer, which has been slightly 
greased, is pulled up (screwing it from side to side to make 
