270 
lin. C. T. Pv. WILSON ON CONDENSATION OF WATER VAPOUR 
the water forms a uniform film over the surface of the glass, instead of collecting 
into drops and preventing a satisfactory view of the interior. 
This apparatus appeared to fulfil very well the purpose for which it was designed. 
Nothing can gain access to the air imprisoned in the inner vessel except by solution 
in and diffusion through the surrounding water. 
The water surface which forms the lower boundary to the space occupied by the 
air under observation, drops suddenly to a new position, where it comes to a sudden 
stop, without any splashing, and remains stationary as long as may be desired. The 
whole movement is certainly over in a small fraction of a second, as the expansion 
appears to the eye to be instantaneous. That it should be rapid is what one would 
expect, as the initial driving pressure is nearly one atmosphere, and the distance 
travelled rarely amounted to more than two centimetres. 
There is this further advantage in such a method of expanding the am, that the 
rate of expansion is most rapid just before it is completed, because the driving pres¬ 
sure still remains considerable, and the water is therefore moving with constantly 
increasing velocity until it brings itself to a sudden stop bv closing the valve E. 
Thus the final stage of the expansion, when the temperature is lowest, and there¬ 
fore the influx of heat most rapid, is that which is most quickly passed through. 
The motion of the water cannot, of course, be stopped instantaneously ; in practice 
it was always found that some small air bubbles were left imprisoned around the 
valve E. These, being compressed by the impact, probably served to diminish con¬ 
siderably the strain on the apparatus. 
With a thin float the volume of these bubbles was quite a negligible fraction of 
that of the air which escaped before the valve closed. If any considerable fraction of 
the air tvere left behind when the valve closed, an error would be introduced by its 
momentary compression, the actual maximum value reached being really greater than 
what is afterwards measured by an amount equal to the momentary diminution in 
the total volume of the air bubble. 
Method of Conducting the Experiments. 
To charge the apparatus with air reasonably free from laboratory gases, the bell- 
jar, with the inner vessel fixed inside it, was removed from the ground-glass plate on 
which it rested, and allowed to remain at an open window for some time. It was 
then placed on the glass plate, and bound tightly down with wire. 
Distilled water, which had been boiled for some time to remove the greater part of 
the dissolved gases, was then poured in till it nearly tilled the bell-jar. By inclining 
the whole, air was allowed to escape from the inner vessel till only a convenient 
volume remained. The bell-jar was then again nearly filled up with water, and the 
apparatus then connected up as already described and shown in fig. 1. 
The glass-plate \vas levelled by means of levelling screws supporting the tripod on 
