152 Proceedings of Poyal Society of Edinburgh. [sess. 
the water is made to flow into the receiver ; when the desired 
number of c.c. have left the vessel and entered the receiver, the 
receiver stopcock is opened, and the water flows out and draws in 
the dusty air, the stopcock T being closed when the water returns 
to its original level. Which of these plans is the best I leave to the 
operator ; but it is always well to vary one’s way of working, as it 
tends to keep us free from errors, and for this reason alone this 
method may be occasionally used. 
The Gasometer. 
The gasometer sketched on Plate I. is of the nsual construction. 
The only points specially attended to are a nice balance, ease of 
movement, and an accurately graduated scale. The scale is in litres 
and percentages. The gasometer has a capacity of 20 litres. 
Attached to the gasometer is the filter F, and inside the gasometer 
is a stirrer for mixing the filtered and unfiltered airs. The gaso- 
meter has a number of duties to perform. It is used as an aspirator 
for circulating the air through the measuring apparatus. It is 
used for testing very dusty air, such as that from flames, when the 
smallest quantity measurable by the apparatus described would give 
a cloud of particles far too dense for counting. The impure air is 
drawn into the gasometer mixed with a known quantity of filtered 
air, and sent through the measuring apparatus to be tested in the 
usual way in the receiver. The gasometer has also been found very 
useful for making check experiments, and for trying the working of 
the different parts of the apparatus. For instance, in comparing the 
results given by different ways of working, it was soon found that 
the number of particles in the outside air was far too variable to make 
it suitable for experiments of this kind. Air with the desired amount 
of dust was, therefore, put into the gasometer and thoroughly mixed, 
and the tests were made with it. Allowance, of course, had to be 
made for the gradual diminution of the dust during the experiments, 
and errors from this cause checked by working the different plans 
alternately a number of times. 
Illuminating the Stage. 
The stage may be illuminated, as shown in Plate I., by means of 
a gas flame ; when gas cannot be had, a paraffin lamp does very 
well. A screen, with an opening opposite the flame, should be 
