40 Proceedings of Royal Society of Edinburgh. [sess. 
structed free from the first four charges, it is probable the fifth 
would vanish. 
First, as regards weight, the experience gained with the portable 
apparatus has shown that the size may be very much reduced if 
the instrument is to be used only for testing air of country districts — 
i.e ., air free from immediate local pollution. Experience has 
shown that in country air the number of particles is rarely over a few 
thousands per cubic centimetre. It is therefore not necessary to use 
the small measures of the portable apparatus under these condi- 
tions, as we can with the air-pump alone test air up to an impurity 
of 25,000 per cubic centimetre. It is ouly for air of greater im- 
purity than this that the stopcock measures are required. These small 
measures may therefore be omitted. Again, when testing country 
air, the proportion of pure air to impure air required to make a test 
does not vary greatly — from 4 to 1 to 19 to 1. The receiver of the 
instrument does not therefore require to be so large as when the air 
to be tested requires to be mixed with some hundred times its 
volume of pure air. This admits of the receiver, and therefore of 
the whole apparatus, being greatly reduced in size. The capacity of 
the receiver in the pocket instrument has therefore been reduced to 
one-fifth of that of the portable apparatus. This at once effects a 
great reduction in the weight, and the stopcock measures not being 
required reduces the expense as well as the weight. 
Turning now to the weak and troublesome parts, these are 
principally : — (1) The air-pump valves are liable to get out of order, 
and occasionally give rise to trouble. (2) The india-rubber tube 
for closing the opening through which the stirrer-rod passes 
occasionally fails, and gives trouble by leaking. Further, these 
india-rubber tubes with closed ends require to be specially prepared 
for the instrument, and it is difficult to get suitable tube for 
the purpose. And (3) the silver counting stages are delicate and 
troublesome to keep. So far as my experience goes this is not the 
case, as after a little practice no trouble has been experienced, and 
the first silver stage put into my instrument is still in use and in 
good condition. My experience, however, is not that of others, 
and it seemed in the highest degree desirable that some improve- 
ment be made in this direction by the introduction of a counting 
stage that could be more easily kept in working order. 
