44 
Proceedings of Royal Society of Edinburgh. [sess. 
air, we must now purify it in another way. To do this the stop- 
cock A is closed, and the stopcock K is turned so as to put the 
pump into communication with the receiver, that is, in the position 
shown in fig. 1. A stroke of the pump is now made. This causes 
condensation to take place on the dust particles when some of them 
drop out of the air. The piston is again put to its top position, 
and another stroke made, when more particles settle, or are 
deposited on the sides by the rush of air. After this expanding 
and condensing process has been done a few times all the particles 
of dust will have become nuclei, and be deposited on the bottom of 
the receiver. The air will now be pure, no drops falling when expan- 
sion is made. This process of purifying the air in the receiver is quite 
as quick as the filtering one. Indeed, when the filtering process is 
used, it is always quicker to end by showers of the last particles. 
The air in the receiver being now purified so that no drops are 
seen falling when expansion is made, the next thing to be done is 
to introduce into the receiver the necessary quantity of the air to 
be tested. However, before doing this, it is advisable to turn the 
stopcock K a quarter turn to the left, so as to bring the receiver 
into communication with the outer air. The object of doing this 
is to bring the pressure inside the receiver to that of the open air. 
When making the repeated expansions to purify the air in the 
receiver, some air may have leaked in past the piston, and it is to 
get rid of this air that the stopcock is opened and the receiver put 
into communication with the outer air before taking in the 
measured quantity. If this was not done the amount measured in 
would be too small by the amount of the leakage. There will be 
no leakage if everything is in good working order ; still it is a 
good precaution always before taking in the air to be tested to turn 
the stopcock and allow any plus pressure to escape. 
The air in the receiver being at the same pressure as the outer 
air, the measured quantity of the air to be tested is then taken into 
the receiver in the following manner : — The piston being at the top 
of its stroke, where it ought always to be at the end and beginning 
of every test, and the stopcock in the position shown in the draw- 
ing, the piston is then drawn down the amount that is thought 
will be suitable under the conditions. Say it is drawn down to the 
mark on the scale, by this movement there is taken out of 
