IN THE PRESENCE OF DUST-FREE AIR AND OTHER GASES. 
273 
In the following table, is the initial volume and the final volume when the 
expansion is just sufficient to cause rain-like condensation. 
Date. 
f C. 
Vi 
ro/wi 
I. 
September 4 
22-0 
292-9 
367-3 
1-254 
2. 
„ 4 
22-4 
293-4 
367-8 
1-2.53 
o. 
„ 5 
28-8 
3I3-I 
392 5 
1-253 
4. 
27-2 
308-5 
385-8 
1-250 
5. 
„ 6 
27-8 
312-5 
.390-8 
1-2.50 1 
6. 
„ 6 
26-0 
309-8 
386-7 
1-248 i 
7. 
„ 7 
24-5 
302-0 
378-3 
1-252 1 
■ 
iMean 
1-252 
The same air was used on September 7 as on the previous day ; otherwise the 
experiments were made on a different sample of air each day. 
It will be noted that the exj^ansion required is sensibly the same at all temperatures 
between 22° and 28° C. Accurate measurements of the initial temperature are 
therefore unnecessary in these experiments. 
The results given in the table show no greater variation than are to be expected 
from the degree of accuracy of the volume measurements. The level of the water 
could be read by means of the telescope to the nearest tenth of a millimetre, 
corresponding to an error of half a cubic centimetre in the volume measurements. 
There may be an error of this amount in the measurement of both and v,, and 
hence an error of 4 units in the fourth figure in the ratio, when the initial volume 
amounts to about 300 cub. centims. 
Other Ex 2 )eriments made luith the same A'p'paratus. 
1. When sunlight was used to illuminate the drops, exactly the same expansion 
was required to bring about visible condensation. 
The result, therefore, does not depend on the kind of illumination used. 
2. Experiments were made to see if the nuclei which cause the rain-like con¬ 
densation could be removed by repeated filtering. For this puiqjose a hole was 
bored through the glass plate on which the apparatus rested. A glass tube reaching 
to the roof of the inner vessel was passed through a cork which closed this hole. 
Through it the air could be drawn out into an inverted Wolff’s bottle, arranged to 
act as an aspirator, and could be driven from the one vessel to the other as often as 
was desired. 
A tightly-packed cotton-wool filter was inserted between the expansion apparatus 
and the Wolff’s bottle. Passing the air repeatedly backwards and forwards through 
MDCCCXCVII.—A. 2 N 
