THE MECHANICAL EQUIVALENT OF HEAT. 
417 
hem during these experiments have already been mentioned on p. 389. We were 
hus restricted to masses of water varying from 130 to 300 grms. 
As the determination of the equivalent and the change in capacity of the whole 
lass due to increase of specific heat are fully discussed in another section, it is only 
ecessary here to deal with the mass of water added to the calorimeter. The balance 
sed by us was by Becker, carrying 500 grins, and turning with a milligramme. As 
his type of balance is not provided with a rider-arm, it was found convenient to add 
.^eights to the nearest 5 milligrms. above and below, and to interpolate by the 
aethod of oscillations. Nearly the whole of the measurements were conducted by 
he ordinary method of direct weighing, as the balance, when once in good adjustment, 
emained so. The adjustment "was however, occasionally checked by the method of 
louble weighing. 
Holes were bored through the bottom of the balance case and the top of the cupboard 
)n which it stood, and wires suspended from the pans for use when weighing flasks, &c, 
\.s it was necessary to open this cupboard whenever a flask had to be weighed, it would 
lave been difficult to keep the air within the cupboard in a state of dessication, and 
,:hus if proper precautions had not been taken a considerable error might have been 
ntroduced in the weighing of glass vessels of large area, owing to the hygroscopic 
lature of their surfaces. The usual simple expedient of taring one glass vessel against 
mother of approximately equal surface appeared eminently satisfactory, as the 
weighings could be repeated at varying intervals of time (generally a few hours, but in 
me test case four days) and the results were always found to agree to a milligramme. 
In our earlier series of experiments (1887-1890) the calorimeter could be rapidly 
taken to pieces, dried, any required mass of water added by means of a fine dropping- 
pipette, weighed, and put together again. As, however, those experiments had shown 
us the necessity of keeping the outer space perfectly dry, we now decided to reduce, if 
possible, the pressure to less than 1 millim., since, in this case, radiation, &c., would 
be considerably diminished, and any moisture present would immediately be detected. 
It thus became impossible to take the calorimeter to pieces, and means had to be 
devised for introducing and withdrawing water through the supporting glass tubes. 
The impossibility of removing the calorimeter without destroying our vacuum led 
^to another difficulty, for, at the end of the experiment, the water had been heated to 
the highest point of our range, and before we could conduct a second experiment we 
had either to cool the whole apparatus down, which would have been an extremely 
slow process, owing to the smallness of the radiation, and would also have rendered 
it necessary to throw the regulator of the outer bath entirely out of adjustment; or 
else withdraw part of, or all, the water in the calorimeter, adding sufficient cold water 
to bring the temperature down below the point at which our observations commenced. 
During the earlier part of our 1891 series we withdrew a portion only of the water by 
means of a modified form of weighed pipette, and then, by the same means, added a 
weighed quantity of cold water. This method was in some respects satisfactory, 
MDCCCXCIIT.—A. 3 H 
