«) J. AITKEN ON THE NUMBER OF DUST PARTICLES IN THE ATMOSPHERE. 



covered with a heavy deposit of dew, in which state it is quite useless, but when mounted 

 in the manner shown, the dew is easily cleared away by heating the tube E, when the 

 heat is carried forwards to the stage by the entering air on its passage from the filter to 

 tin 1 receiver. It will be noticed that the stage is not placed centrally over the pipe E. 

 This arrangement has been adopted because it has been found to keep the stage in the 

 best working condition. If the stage is too hot, the drops roll away and quickly eva- 

 porate ; on the other hand, if it is too cold, the surface gets wet, and counting becomes 

 impossible. It has been noticed that the entering air tends to cool the stage, and it is 

 found better to confine the cooling to one side of the stage, because, by this arrange- 

 ment, thereis gene rally some part of it at the temperature most suitable for easy count- 

 ing ; and, as only one or sometimes four little squares are required, there are always 

 plenty to select from in the hundred on the stage. The stage is viewed through the 

 bottom of the flask, by means of the compound magnifying glass S. A common cheap 

 instrument, to be had from most opticians, has been found to be quite suitable for this 

 purpose. 



The pipe C, connecting the pump with the receiver, terminates inside the receiver in an 

 enlargement P, which is packed with cotton wool for a reason which will be explained 

 later on. The stage is illuminated by means of the gas flame R, the light from which 

 passes through the spherical flask Q, which is filled with water. This spherical lens sifts 

 the heat out of the rays, and concentrates the light on the stage. T is a screen enclosing 

 the receiver A, to cut off all light except what is concentrated on the stage by the lens 

 Q, and it is blackened inside to check reflection. 



Suppose now the apparatus fitted up as shown in the figure, and that we wish to test a 

 sample of air, the first thing to be done is to close the stopcock H, and open F. The 

 pump B is now worked, and air drawn through the filter D, this is done for some time 

 till the air begins tog etpurified; or most of the air may be pumped out and filtered air 

 allowed to enter. After this has been done the stopcock F is closed, and a stroke of the 

 pump made ; condensation then takes place inside the flask, and if the air is nearly pure, 

 the drops are large and soon settle down. After they have fallen, the stopcock F is again 

 opened, and filtered air allowed to enter, when it is again closed, and another stroke is 

 made with the pump. This is repeated till all condensation stops in the flask. The air 

 inside the receiver is now free from all dust particles. The stopcock F is then closed, the 

 tube K disconnected from M, and the stopcock H withdrawn from its air-tight fitting L. 

 The flask G being thus thoroughly disconnected from the apparatus, its stopper is now 

 removed, it is filled with water, and carried to the place from which we wish to take a 

 sample of the air to be tested. Here the water is nearly all emptied out of the flask, and 

 the air enters to take its place. The stopper must now be tightly fitted in, the stopcock 

 closed, and the pipe K filled with water and clamped. Returning with the flask to the 

 apparatus, the stopcock H is carefully fitted into its place, the tube K connected with 

 M, the lower part of M being full of water, and care taken that no air gets shut in between 

 the water in the tube and the water in M. The stopcock H is now opened, and water 



