MR AITKEN ON ATMOSPHERIC DUST. 489 



others, as observations of it were made at a considerable elevation, whilst the others 

 were taken near sea level. It may be as well to note here, that as Dr Fredlander and 

 Professor Melander used the same instrument for counting the dust particles as I did, 

 namely the pocket dust-counter, it is therefore not necessary to make any allowance 

 for instrumental differences. 



There are still many questions connected with atmospheric dust waiting an answer, 

 such as its effect on the temperature of the air ; how far it absorbs the sun's heat 

 during the day, and radiates its own heat during the night. There are some interesting 

 points connected with radiant heat and dust. It seems somewhat surprising that the 

 large visible dust motes in the air should be able to pass unaffected through the focus 

 of a lens concentrating the sun's heat, where a larger body would be burned up ; but 

 the motes take no notice of it, not even seeming to be heated to any extent, as there 

 is no sign of an ascending current of air at the focus, a result one might expect from 

 the freedom with which very small bodies part with their heat to the air. Then again, 

 there are the sources of the supply of the dust particles. Many of these are well known, 

 and perhaps the remains of the meteors dissipated in the atmosphere would keep up a 

 sufficient supply for the formation of clouds ; but the dust produced by the action of 

 sunlight on certain gases in the atmosphere has only been studied to a small extent, 

 and a few of the gases indicated which in sunlight give rise to condensation nuclei. 

 Another question is, how high and how far do these particles go ? It is hoped that the 

 observers going to the Arctic and Antarctic will be able to make a full series of dust 

 observations in these regions, as any dust observed, when not due to volcanoes, must 

 either be of cosmic origin or have been carried from far away inhabited areas. Dust 

 observations there might help to throw some light on its source, as we would expect 

 the variations in the amount of terrestrial dust would be dependent on the direction 

 of the wind, whilst the cosmic would not. 



ROY. SOC. TRANS. EDIN. VOL. XLI1. 3 R 



