46 
Proceedings of Royal Society of Edinburgh. [sess. 
trial, less than one drop per square millimetre fell, then the 
quantity of impure air ought to be increased to, say, impure air. 
Sometimes, however, the air is so pure that i is too little, and it is 
desirable to have no pure air in the receiver, and to fill it entirely 
with the air to be tested. When this is the case, the stopcock K 
is turned so as to put the receiver into communication with the 
outer air, and the air is drawn out of the receiver through the stop- 
cock A. This may be done either by means of the mouth, or by 
any simple piece of apparatus. The current must be kept flowing 
through the receiver till all the pure air has been drawn out. After 
this the stopcock A is closed, the receiver put into communication 
with the pump, the stirrer worked, expansion made, and the drops 
counted in the usual way. When working in this way the number 
obtained per cubic centimetre in the air of the receiver has to be 
multiplied by 1*4 to allow for the reduction in number produced by 
the expansion. When working in pure air it is often necessary, 
instead of confining the attention to one square millimetre, to 
observe the number of drops that fall on a square of four squares, 
or on a square of nine squares, that is, of nine square millimetres. 
Having described the manner of working the new apparatus, we 
shall now proceed to describe what has been done to improve the 
counting stage, and make it more simple and easily kept in working 
order. Naturally glass seemed the most suitable substance for 
making those stages on account of the perfection of its surface, as 
well as for the ease with which it can be kept clean. I had pre- 
viously tried glass, but with no good results ; but though I had 
hitherto failed, the many advantages to be derived from the use of 
glass induced me to make a fresh attempt. The difficulty with 
glass is that the drops when they fall on it are nearly invisible. It 
does not matter whether we use glass mirrors or blackened glass — 
in all cases it is difficult to see the drops. On examining into the 
cause of this difference between glass and silver surfaces, water 
spray was allowed to fall on these surfaces, and the drops were then 
examined by means of a magnifying lens as they rested on the 
different surfaces. It was seen that on the silver, the drops scarcely 
touched the surface, but formed little flattened balls, and their 
brilliancy is due to the light reflected from the internal concave 
surface furthest from the light ; whereas the drops on glass adhere 
