8 J. AITKEN ON THE NUMBER OF DUST PARTICLES IN THE ATMOSPHERE. 



so to speak, diluted 450 times. But the air is not only diluted, it is also expanded by 

 the pump, which has a capacity of 150 c.c. The dust that was in our original 1 c.c. is 

 thus expanded by the two processes into 600 c.c. The number of drops per c.c. counted 

 on the stage must therefore be multiplied by GOO to give the number in the 

 original cubic centimetre of dusty air. Suppose that we counted one drop per square 

 millimetre, then, as there is 1 cm. of air above the stage, this will give 100 drops per 

 cubic centimetre in the diluted and expanded air, and this multiplied by 600 gives 

 60,000 dust particles per cubic centimetre of the air tested. 



With apparatus of the proportions shown, the described method of working is suitable 

 for air in the condition usually found outside, but it will not do for such air as is found in 

 rooms where gas is burning — 1 c.c. of air of this kind when expanded only 600 times 

 would give such a dense shower of drops that it would be impossible to count them. We 

 might, of course, put in less than 1 c.c. of the air to be tested, but it is evident that 

 accuracy cannot be obtained if very small quantities are used, because it is difficult to 

 measure them correctly owing to variations in temperature and pressure, and also 

 because of the imperfections of the apparatus. The tubes connecting the stopcock H are 

 made of as small diameter, and as short as possible ; yet, it is evident that, however 

 small we make them, they will always hold some air, which, by the construction of the 

 apparatus, is counted in the measured air ; but owing to the smallness of the tubes this air 

 will not have so much dust in it, as much of it will settle out while standing. 



A better plan for testing very dusty air is, first to fill the flask G with water, attach it 

 to the apparatus, and after purifying the air in the receiver, open the stopcock H and 

 disconnect K from M ; then syphon out a measured quantity of water, say 400 c.c, allow- 

 ing filtered air to enter by H to take its place. The flask G is then disconnected from 

 E, and 40 c.c. of water syphoned over, while its place is taken by the air we wish to test, 

 which is allowed to enter by the stopcock H. The rest of the testing is made as before ; 

 and the calculations are all as in the previous example, only multiplied by 10, or whatever 

 the proportion was in which we mixed the dusty air with pure air in the flask G. 



Another method of working, and the one which was principally used in the pre- 

 liminary work, is somewhat different, and seems to possess some advantages over the 

 one described ; the apparatus, however, is much larger. In this other method the flask G 

 and the filter D are removed, the rest of the apparatus remains as shown, and in place of 

 the flask a gasometer is used. The gasometer employed has a capacity of 20 litres ; it is 

 delicately hung, and accurately graduated, so that air can be measured in it to an accuracy 

 of about yjtq litre. The gasometer is provided with a large filter 5 inches in diameter 

 and 5 inches deep. The manner of working is as follows : — Back weights are put on, and 

 air drawn in through the filter till the gasometer is nearly full. A measured quantity of 

 the air to be tested is then added to it, and the mixture stirred by means of a metal disc 

 which can be moved up and down inside the gasometer. 



When testing the outside air, it is introduced into the gasometer by means of a pipe, 

 one end of which passes to the open air, while the other is attached to the gasometer. 



