THE MECHANICAL EQUIVALENT OF HEAT. 
419 
we found, although we had, as we thought, dried completely, there still remained 
several cub. centims. of water. We subsequently ascertained that the calorimeter 
could be properly dried in the above manner, provided that sufficient time was 
allowed; but the hot air had to pass for several days before a drying tube, suspended 
within the calorimeter for 12 hours, showed no increase in weight. Now, as the first 
method involved too many weighings, and the second was unsafe, unless two or three 
days elapsed between each experiment, we were compelled to seek a different mode of 
procedure. Thus, as we could neither withdraw any portion of the water, nor cool 
the whole calorimeter down without great labour, means had to be devised for cooling 
it from within. A tube, through which a freezing mixture could be forced, was 
evidently objectionable, as there would always be uncertainty as to whether all the 
freezing mixture had been removed. We found the following arrangement worked 
admirably, for the temperature of the calorimeter fell as rapidly when cooling, as it 
rose when the current was passing. We were thus able to use the same water again 
and so save a numerical reduction. 
A very thin glass tube, closed at its lower end, passed down the support tube F 
(Plate 2, fig.l) of the calorimeter, just clearing the base, and the upper end, which was 
open, projected about ^ inch above the top of the tube F. Gare was taken to so fix this 
tube that no water could find its way up the annular space between the two tubes. 
When not in use, the upper end was closed by a cork, so as to prevent air-currents 
circulating down to the calorimeter. When used for cooling, this tube was partially 
filled with ether, through which a rapid current of dried air was sucked by means of 
the arrangement shown in Plate 2, fig. 1. The fine capillary point reached just to the 
bottom of the closed tube, so that, when the cooling had proceeded far enough, the 
remaining ether could be withdrawn by reversing the connections to the exhaust pump 
and th^ H^SO^ drying bottles. Any ether or impurity left at the bottom of this tube 
would have caused great irregularities in the experiments, so, as a precaution, only 
freshly distilled ether was used,’* and when the temperature had been sufficiently 
lowered, dry air was passed for a considerable time and, finally, the cooling tube 
thoroughly cleaned by means of an absorbent mop. The joint at G was made by slipping 
a small ring of india-rubber tubing round the projecting end of the cooling tube, and, 
by slightly splaying out the mouth of the aspirating portion, a sufficiently good union 
was effected, the extreme thinness of the walls rendering a ground glass joint impractic¬ 
able. The entrance tube K was used for adding fresh ether as occasion required. 
As soon as we perceived that this method of cooling was a success, it was at once 
evident that it could be applied to finding the latent heat of evaporation of liquids, 
for by regulating the rate of evaporation we could make the cooling effect such as to 
counterbalance the heat developed by a known current. We have already made a few 
rough determinations, but prefer to postpone their publication until we have applied 
the method with greater precautions. 
* The ether was always redistilled by us immediately before use—at a temperature under 40° C. 
3 H 2 
