430 PROFESSOR A. SCHUSTER AND MR. W. GANNON ON A 
may introduce errors into a calorimetric measurement. With French or German 
standard glass the depression of the zero, corresponding to a certain interval, is less 
than the thousandth part of that interval. If the thermometer has been, before the 
experiment, sufficiently long in the calorimeter to take up its permanent position, 
and if the rise of temperature is then made to take place, the position of the zero 
in the final period must lie between its original value and that corresponding to 
permanent immersion in the final temperature. The error made by either assumption 
cannot exceed one part in a thousand, and we may form a sufficiently good estimate 
as to the actual zero value to reduce considerably the limits of possible mistake. 
But the slow set of the glass envelope which shows itself in the shifting zero, may 
affect our results in another way. The correction due to loss of heat by radiation and 
conduction is generally determined by observing the temperature of the calorimeter 
at the beginning and conclusion of the experiment. But even if there is no loss in 
the final stage, the thermometer will apparently fall slowly, owing to the imperfect 
behaviour of the glass envelope. Let the rate of fall be 8; if the calorimetric 
observations are conducted in the usual way we should over-estimate that loss 
by the quantity 8. If the time during which the calorimeter has been rising in 
temperature is 7, we may correct for the error made in this way by adding 4r to the 
final temperature, or deducting $¢ from the zero. This is equivalent to saying that we 
may correct for the set of the glass envelope in a calorimetric measurement by taking 
for the zero during the final stage the zero shown by the thermometer after immersion 
in the final temperature during a period equal to half the length of time it has taken 
the thermometer to rise from its initial to its final value. In our experiments this 
would be between four and five minutes. 
To obtain some clearer information as to time taken by our thermometers to arrive 
at this permanent zero a series of experiments were carried out. We had a thermo- 
meter (Baudin, 12773) which in all respects was as nearly as possible like the one 
used in our calorimeter experiments (12772). The former was placed in water of 
about 22°, and kept there for over four hours, the latter was kept during the same 
time at a temperature of from 10° to 12°, and then suddenly raised to the higher 
temperature and placed side by side with No. 12773. The two thermometers were 
read alternately every quarter of a minute for about a quarter of an hour, so as to 
obtain a good comparison. After the lapse of another hour a similar set of readings 
were taken at the same temperature. As the mercury threads stand between the 
same divisions in the two cases, a systematic difference in the reading cannot be due 
to any calibration errors. The following Table III. gives. the results of the com- 
parisons. The times are given in minutes reckoned from the moment that 12772 
was placed beside 12773. The column headed A denotes the difference between the 
indications of the two thermometers, 12772 always reading higher. 
