J. AITKEN ON THE NUMBER OF DUST PARTICLES IN THE ATMOSPHERE. 5 



Description of Apparatus. 



Having given a general outline of the method employed for counting the particles, I 

 shall now describe more particularly the apparatus first used in this investigation. The 

 general arrangement of the apparatus is shown in the Plate. All the apparatus is of the 

 simplest kind, and can be easily obtained ready made, with the exception of the small 

 stage on which the drops are counted. The different stages used in this investigation 

 were ruled by myself with the aid of a simple instrument constructed for the purpose, 

 but which need not be described here. In the figure A is the test receiver into which the 

 air under investigation is introduced, and inside which the particles are counted. It is 

 an ordinary glass flask with flat bottom, and supported in an inverted position. B is an 

 air-pump connected with A by the india-rubber tube C. It may be mentioned that the 

 pump B is drawn in the position shown in the sketch for the convenience of illustration. 

 In the actual apparatus it is fixed horizontally a little above the level of the table, as this 

 position has been found to be the most convenient for making a stroke of the pump while 

 the eye is kept steadily applied to the magnifying glass. D is a cotton-wool filter con- 

 nected with A by means of the pipe E. The pipes C and E pass through an india-rubber 

 stopper in A, and project upwards into the receiver; C stops about the middle ; while E 

 rises to near the top, and forms the support to which the counting stage is attached. 

 F is a stopcock for closing the connection between the receiver A and the filter D. 



The air to be tested is introduced into the flask G. This flask is provided with an 

 india-rubber stopper, through which pass the two tubes I and K. The tube I enters only 

 a short distance into the flask, and is provided with the stopcock H. The upper branch 

 of this stopcock communicates with the tube E by means of the movable air-tight joint 

 L. The other tube K attached to the flask G passes downwards to near the bottom of 

 the flask, while outside it is connected with the small glass vessel M in the manner shown, 

 by means of a length of india-rubber tube. The vessel M is made large enough to hold 

 a few cubic centimetres of water, and at the lower end, at the narrow part, is an engraved 

 index line. Above the vessel M is supported the burette N. 



To keep the air under examination saturated with vapour, some water is put into the 

 receiver A, and from time to time the receiver is inverted, and the water moved about 

 inside the flask so as to wet the inner walls. To enable this to be done easily, there is 

 a break in the metal tube E, just below the stopcock F, which is joined by means of a 

 short length of india-rubber tube, so that when the ring which supports the receiver is 

 removed, it can be turned about in any direction to bring the water inside in contact with 

 all parts of the interior. 



The stage on which the drops are counted, as already explained, is made of a small 

 plate of highly polished silver ; it is a little over 1 cm. square, and ruled with very fine 

 lines at right angles to each other and at 1 mm. apart. The stage is supported inside 

 the receiver A by means of the pipe E at exactly 1 cm. below the flat top of the receiver. 

 The object of fixing the stage to the top of the entrance pipe is that the stage often gets 



