262 MR JOHN AITKEN ON THE 



as high as it safely could be, to cleanse the instrument thoroughly ; after which, 

 as we know, it will cease to give off nuclei at a lower temperature. When the 

 tube was thoroughly cleansed by means of heat, and all the impurities swept 

 out of it by a current of air, the temperature was lowered slightly, and the air 

 allowed to pass slowly through the tube on its way to the test-flask. After 

 this, the fogging in the flask gradually diminished, and after passing through 

 the rainy stage, it ceased entirely, proving that the filter was doing its work 

 thoroughly, not a single particle— not even one of the very minute and invisible 

 ones— escaping it. On equalising the temperature, either making both tubes 

 hot or both cold, the filtering action of course ceased. 



It does seem somewhat strange that air should be freed from all its 

 dust in passing through a channel large enough for a fly to pass, if it has 

 sufficient intelligence to keep always on the cold side. All who have 

 experimented on this subject know that dust can get through any opening, 

 however small. On testing this filter for the first time, I failed to get a satis- 

 factory result. I however felt convinced that it ought to work, and the 

 failure was attributed to some imperfection in the tubing or joints. Arrange- 

 ments were therefore made for testing the tightness of the whole apparatus. 

 The one end of the filter being connected, as described, to the glass flask in 

 which the air was tested, I now connected a cotton-wool filter to the other 

 end of the thermic filter, and proceeded to test if all was tight, by drawing in air 

 from the cotton- wool filter through the apparatus, while it was cold. At first, 

 I could not succeed in getting air free from dust-; fogging always took place on 

 reducing the pressure in the flask, showing that dusty air was leaking in some- 

 where, and mixing with the filtered air. After much time spent in remaking 

 all the joints, it was discovered that the air-pump valve was not quite tight ; 

 by allowing the leakage to bubble through the water in the flask, it was found to 

 be very slight, only about 2 or 3 c.cm. per minute. After this was put right, 

 fogging still appeared, showing that there was still leakage. This time it was 

 traced to the stop-cock between the filter and the test-flask. This leakage was 

 smaller than the other, yet it let in dust. After all leakages had been stopped, 

 the cotton-wool filter was removed, and the thermic filter being heated, was now 

 found to do its work satisfactorily, though more slowly than a cotton- wool filter. 

 The ease with which dust passes through small openings is surprising ; indeed, 

 I have found that any opening which admits air, also allows these less than 

 microscopic particles to pass, and yet the air in its passage through the wide 

 channel of this filter had every particle of dust taken out of it by the thermal 

 conditions to which it was subjected. 



If we cause the filter to purify air into which we have intentionally put a 

 good deal of dust, such as dust of calcined magnesia, we find all the dust 

 collected on the surface of the cold tube, near the end where the air entered, 



