170 
Proceedings of Royal Society of Edinburgh. [sess. 
The diaphragm, as in the other instrument, is moved by a rod, 
surrounded by an indiarubber tube. This rod is not shown in the 
drawing, owing to its position being at right angles to the section ; 
A is the air-pump, which has a capacity of 15 c.c. ; the piston- 
rod is graduated in this case to its displacement, as shown ; S is a 
stopcock, bored as shown ; M is the apparatus for measuring the 
air to be tested, while F is the filter. 
It will be noticed, that in this instrument we have adopted the 
stopcock plan, for measuring small quantities of air, which was de- 
scribed in the first part of this paper. The method of working has, 
however, been so far improved that it is not necessary by the new 
arrangement to change the measuring apparatus when we require to 
use a larger or smaller quantity of dusty air. As will be seen, the 
different measures are arranged in series, and we can use any of 
them without making any alteration in the apparatus. 
In this portable apparatus, as in the other one, the pump is used 
when we require to mix large quantities of dusty air with the air in 
the receiver, and the measure M is used for small quantities. If 
1 c.c. of dusty air is required to be sent into the receiver, then, after 
sufficient time has been allowed for the filtered air to enter the 
receiver, and when the pressure inside the receiver is the same as 
outside, but not before, the stopcock is turned quarter a turn from the 
position shown, by this means the receiver is put in direct con- 
nection with the outer air. The piston of the air-pump is now 
drawn down to the figure 1 on the scale ; by this means 1 c.c. of the 
air is taken into the receiver, where it is mixed with pure air and 
tested. Two, or perhaps three, cubic centimetres may be measured 
in this way; but if more is required, then the stopcock S must be 
closed before the piston is drawn down, otherwise some of the dusty 
air might be taken out of the receiver by the pump, the piston 
of the pump should be pushed back to the top before the stopcock 
is opened to admit the air. When the stopcock is closed while the 
pump is used as a measure, allowance must be made for the expan- 
sion of the air, as already explained.* 
* After a considerable amount of practice with this apparatus, I find it 
better always to close the stopcock before drawing down the piston, even when 
only 1 c.c. of air is required. The graduation of the pump ought therefore to 
be made to show the amount of air taken out of the receiver while the stop- 
cock is closed. 
