154 Proceedings of Royal Society of Edinburgh. [sess. 
wet the sides of the receiver and the paper covering the top 
along its outer edges. If the outer edges get wet, the water soon 
spreads all over it. The necessity for an occasional wetting inside 
explains why the receiver is connected with the measuring apparatus 
by means of a piece of indiarubber tubing. The disc I can be easily 
wetted from time to time by drawing it down into the water beneath 
it. The air in the receiver is now purified by pumping in air 
through the filter, and finally by expansion and consequent throwing 
down of particles by showers of rain, filtered air being admitted after 
each shower. If all the joints are air-tight, condensation rapidly 
ceases, and after a few showers not a drop will be seen falling when 
expansion is made. The counting stage should now be attended to. 
If it is dewed, then the mouth of the operator should be applied to 
the tube U, and warm air blown through it. This rapidly warms 
the stage and clears it. If the stage should be troublesome to keep 
clear of dew, it is an indication that the water in the receiver is too 
cold ; that is, too cold for the temperature of the upper part of the 
receiver. Some water ought therefore to be run out, and some slightly 
heated water added to that in the receiver. Too much hot water 
must not be added, or the drops rapidly evaporate before they can 
be counted. The best condition seems to be when the stage tends 
slightly to get dewed, and requires an occasional blow through the 
tube U to clear it. In that condition the drops remain visible some 
time, and are easily counted. 
Supposing the air to be perfectly pure, and the stage in good 
working order, we may then proceed to testing the air. The first 
thing done, after seeing that the air is circulating through the 
apparatus, is to close the receiver stopcock, and then make one 
stroke of the air-pump, and push the piston back to the bottom 
position. A quantity of the air to be tested is then measured, say 
by the small cylinder measure, and sent into D. The receiver 
stopcock is then opened, and the air rushing in from the filter, 
carries the dusty air with it into the receiver. After this is done 
the receiver stopcock is closed, and the stirrer I is rapidly moved up 
and down two or three times. This is done by grasping the lower 
end of the indiarubber tube M and moving it up and down ; the 
indiarubber tube expands and contracts, and allows the stirrer to be 
worked as easily as if it were not inside an air-tight receiver. The 
