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



evident If it were uot for my note-book and the interval of time which has 

 elapsed since the preliminary trials were made with the apparatus, — time which has 

 been spent in clearing these obstructions, — one would be unconscious that they ever 

 existed. As, however, these same difficulties will probably occur to any one who may 

 be induced to follow this line of investigation, it is advisable that some reference be made 

 to them, and the manner in which they may be avoided pointed out. 



When the different tests of the same air gave such different results, it was at first 

 thought that the difference might be due to imperfect filtering. In the process described 

 we take an amount of filtered air and mix it with a small amount of the dusty air we 

 wish to test, and we assume that all the centres of condensation were dust particles con- 

 tained in the dusty air. Now, it is very evident, that if the pure air is imperfectly 

 filtered, our results will not be true, and the numbers will vary with the more or 

 less perfect filtration of the air. As then the filtering of air lies at the very root of 

 the matter, it was thought advisable to study this process more in detail, and determine 

 whether it is possible by means of cotton wool to filter out all the dust particles, and, if 

 possible, to determine the conditions under which this can be done. 



The apparatus used for studying this question was much the same as that represented 

 in the Plate. The flask G was removed, and the pump and the filter connected with the 

 receiver in the manner shown ; the connecting pipes, however, only entered the receiver to 

 a height a little above the level of the water, so as to leave the interior of the receiver 

 clear for observing any condensation that might take place. The gas flame was brought 

 down to very nearly the level of the receiver, the light being allowed to pass through the 

 opening in the screen U, the screen being painted black on the side next the receiver. 

 By this arrangement a strong light was thrown into the receiver by means of the water 

 lens, and by taking up a position with the eye a little above the level of the receiver, the 

 light was not seen, while the illuminated drops were clearly observed against the black 

 background. Some other little black screens were also introduced to cut off all light 

 except that coming from the drops, but these need not be particularly referred to, as each 

 operator will adjust them to suit himself. 



The filters consisted of cotton wool packed into glass tubes, similar to that shown at D 

 in the figure, only of different lengths. The internal diameter of these tubes is 1 inch. 

 A plug of wire-cloth was fitted into the bottom of each tube for the wool to rest on and 

 prevent it pressing into the small exit tube. 



The filtering powers of different lengths of closely packed cotton wool were tested 

 in the tubes, beginning with a length of 1 inch, and gradually increasing up to 12 

 inch (30*5 cm.). There appeared no very definite method of measuring the effect of 

 different lengths, as, in addition to the tightness with which the wool is packed, the 

 filtering power depends on two things. First, the thickness or length of filter ; and 

 second, the rate at which the air is passed through it. The following is the method 

 employed for testing the filtering powers of tightly packed cotton wool, and will give 

 an idea of the filtering power under different conditions : — The receiver employed has a 



