1888-89.] Mr John Aitken on Dust Particles. 169 
Part II. — Portable Apparatus. 
(Read March 18, 1889.) 
When working with the apparatus described in the beginning of 
this paper, it soon became evident that, though it was suitable for 
laboratory work, it was yet very inconvenient when observations 
had to be made on the air of places at a distance — first, because of 
its size ; and second, because it required a house of some kind in 
which it could be fitted up and worked. Further, in making tests 
in a house, there is always the difficulty of local contamination. 
We may select the place of observation as carefully as possible, yet 
the house we select can almost never he in a situation towards which 
the products of combustion from some neighbouring house, or even 
from the selected house itself, may not he driven by some direction 
of wind. As a result of this, we may not he able to make any tests 
of the air when the wind is from certain directions. 
It therefore became desirable, for testing the pure air of the 
country, and for examining the air at places at a distance, that some 
portable form of the apparatus he constructed — one which could he 
easily carried by one person, and which could be worked in the 
open air, so that the observer might carry it to a situation some- 
where to the windward of human habitations or other sources of 
pollution. With this object in view, I have prepared designs of 
a portable instrument which can be packed into a small space, and 
be easily carried. 
The portable apparatus is shown in Plate IV., which is drawn full 
size. As will be seen, the instrument is simply a rearrangement of 
the apparatus already described, but on a smaller scale, and with 
some slight modifications. All the sizes in this apparatus are reduced 
to xg-th the size of the one previously described. In Plate IV., R is 
the receiver, in this case made of brass with a glass cover. Part of the 
glass cover, the two sides of the diaphragm, and the bottom are covered 
with blotting-paper cemented on them. This paper is used for 
holding the water required for saturating the air tested in the 
receiver. The receiver has a capacity of 35 c.c., exclusive of the 
space occupied by the wet blotting-paper. The stage, the inlet and 
the outlet pipes, all occupy the same positions as before ; the dia- 
phragm, however, in this case, when stirring the air, has to be 
moved in the space over the counting stage, and not below it. 
