1888 - 89 .] Mr John Aitken on Dust Particles. 
147 
some dust will have got into the air in C, which will show when 
this air is sent into the receiver and expanded. 
Having explained the action of the apparatus when measuring 
quantities from a fraction of a cubic centimetre to 10 cubic centi- 
metres, I shall now explain its action when larger quantities have 
to he measured. In this case the pump connected with the receiver 
is used as the measure. When dealing with large quantities of 
dusty air, the first thing to he done is, as before, to purify the air in 
the receiver. To do this the stopcock U is opened, and F has its 
handle against the stop H r After the air is purified the stopcock 
U is closed, and F brought up against H 3 . The receiver stopcock 
is closed, a stroke of the pump made, the piston pushed back, and 
the stopcock W closed. On now opening the receiver stopcock, air 
from A rushes in, and from the diameter of the air-pump and the 
length of stroke we have made, we can calculate the number of cubic 
centimetres of air that have entered the receiver. We shall, however, 
refer more particularly to this method of working later on. 
We now come to the second plan of measuring the dusty air. 
This is shown in Plate III. fig. 6. As will be seen, it consists simply 
in the- use of a four-way stopcock as a measure, and the quantity 
measured depends on the capacity of the hole in the plug. The 
stopcock shown in the figure is the smallest of the set belonging to 
my apparatus. As it was to be used for small quantities, two mea- 
sures of different capacities were put into the one plug, but the larger 
measures have only one. The air to be tested is drawn through AA. 
The port B is connected with the test-receiver, while the port C is 
attached to the filter D. In using this apparatus one of the ports 
A is connected by means of a pipe with the source of the air we 
wish to test ; the other is attached to an aspirator. When the plug 
is turned so that the bore is horizontal, as shown in left-hand figure, 
the aspirator draws the air to be tested through the stopcock, and 
the bore gets full of the air we wish to test. While this is taking 
place, one stroke of the receiver air-pump is made. When the 
measuring stopcock is now turned with the bore vertical, as in 
right-hand figure, the air rushing up from the filter carries the plug- 
ful of dusty air along with it into the receiver. Knowing the 
capacity of the plug, we know the amount of dusty air that has 
entered the receiver. In the measuring stopcock sketched, the larger 
