CHANGE IN WEIGHT DURING CHEMICAL REACTION. 
245 
of reaction vessels was made and charged as in the earlier experiments, and their 
volumes equalised as before. After the usual preliminary cleaning and wiping, the 
vessels were suspended from the balance, the necessary small weight added to one of 
the pans and the beam fatigued. During the first series of determinations which were 
then undertaken, the weighings were conducted in the presence of the ordinary air ; 
but the second and third series of weighings were efiected in the presence of dried and 
purified air only. In using the purified air, the streams were allowed to flow 
continuously until just before the actual weighing was commenced when they were 
temporarily arrested by closing the tap t. The first and third series of determinations 
each extended over a period of 7'5 hours; the second series occupied a period of 5"5 
hours. The final results obtained are given below :— 
Table IV. 
Date. 
Experi¬ 
ments. 
Air used. 
R.Ps. reduced for 22° C. 
Number 
of deter¬ 
minations. 
Mean. 
Maximum. 
Minimum. 
Maximum 
difference. 
August 21, 1911 
Series 1 
Ordinary 
106-1 
108-3 
105-1 
3-2 
15 sets of 3 
22 
» 2 
Purified 
104-8 
105-3 
104-1 
1-2 
11 „ 3 
„ 23 „ 
„ 3 
3? 
104-7 
105-3 
104-2 
1-1 
7 „ 3 
In connection with the above table two points may be noted, (a) In Series 1, the 
mean value for the reduced R.P. is 1'3 greater than the corresponding value deduced 
from Series 2 and 3 ; it is believed that this diflerence is due to a probable want of 
equality in the areas of the two vessels. Our data showed that the reaction vessel, to 
which a small volume compensator had been attached, suffered the loss of weight indicated 
by the change in the P.P. ; this is, as we have already shown, exactly what we should 
expect; for, generally speaking, we find that the volume compensator causes the area 
of the compensated vessel to exceed the area of its companion, {h) Notwithstanding 
the very considerable additional refinements that had now been introduced, there still 
remained some residual disturbing factor, the magnitude of which was approximately 
represented by the extreme variations in the P.P., observable in Series 2 and 3. 
Doubtless a portion of the small variation was due to unavoidable experimental error ; 
but at least another portion was probably due to a cause which we shall now proceed 
to discuss in the next section. 
(6) y. Of Errors Arising from Slight Variations in the Temperature of the 
Contents of a Vessel .—Coming now to a brief consideration of the influence exercised 
by the third variable (y, p. 236), we note that some workers have been somewhat 
perplexed and exercised in their minds as to what material constitutes the most 
appropriate substance for wiping glass vessels that are to be weighed; some advocate 
the use of silk, others are in favour of the use of old and fine linen, others again 
