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



Estimated Number of Particles in Airjrom different Sources. 



Having described the method of counting, the apparatus employed, and some of the 

 precautions necessary in conducting the tests, I shall now give some of the numbers 

 obtained by this method of counting. These numbers have all been taken with the use 

 of the gasometer instead of the flask G shown in the Plate. Comparative tests have been 

 made with the gasometer and flask arrangements, but not sufficiently extensive to give 

 accurately their relative values. The numbers given by the flask method are generally 

 less than with the gasometer, owing to the particles settling more quickly in the small 



flask than in the gasometer. 



Number of Dust Particles in Air. 



Source of Air. 



Number per c.c. 



Number per c. in. 



Outside (Raining), 

 Outside (Fair), . 

 Room, .... 

 Room near ceiling, . 

 Bunsen flame, . 



32,000 



130,000 



1,860,000 



5,420,000 



30,000,000 



521,000 



119,000 



30,318,000 



88,346,000 



489,000,000 



In the first column of the above table is entered the source of the air tested. In the 

 second column the number of particles per cubic centimetre ; and for the benefit of those 

 who prefer English measures, there are entered in the third column the numbers per 

 cubic inch. 



The experiments made on the air of our atmosphere are too few as yet to enable 

 us to draw any conclusions from them. But, so far as they go, they indicate that 

 there is most dust in the air during dry weather, and perhaps during anticyclonic con- 

 ditions, and least during wet weather, and perhaps in cyclonic areas. The first number 

 entered in the table for outside air was taken on the 25th January, after a wet and 

 stormy night. The next number is an average for dry days, while we had anticyclonic 

 weather. It is hoped that, by more extended experiments, and by the use of the 

 improved apparatus now under construction, some interesting meteorological results 

 may be arrived at. 



The air in the laboratory was tested in the first case at a height of 4 feet from the 

 floor, and gave the third number entered in the table. Air drawn from near the ceiling 

 gave the fourth number. The reason of the greater number of particles in the room 

 than that found outside was due to the particles produced by two gas flames burning 

 in the room at the time. The air from the Bunsen flame was collected by means of a 

 small chimney, and drawn direct into the gasometer, where it was mixed with filtered 

 air. So full was this air of particles that it had to be mixed with 80,000 times its volume 

 of filtered air before the particles were separated widely enough for counting. All the 

 numbers in the table are far from being constant ; they are all found to vary much 



