t)»'>4 MR JOHN AITKEN ON THE NUMBER OF DUST PARTICLES IN THE 



possibly have been due to local pollution caused by an increase of household fires during the 

 dav. These Kingairloch abnormal readings, therefore, stand by themselves, and may be a 

 purely local phenomenon, or due to some cause of which at present we know nothing. 



In Part II. it was shown that there was generally much dust when there was much 

 sunshine at Alford ; but on the sunny days at Alford the number was high morning, noon, 

 and evening, whereas at Kingairloch the number was only high after the sun had been 

 shining some time, and often not till the afternoon. It will, however, be necessary to 

 keep the abnormal Kingairloch observations in view in the future, as, if the sun have any 

 such effect, it will modify our views regarding the amount of dust on the shores of the 

 Mediterranean, where there is so much sunshine, and on the daily maximum on mountains, 

 which may in part be due to this cause, as it is most marked on sunny days, which heat 

 the air and cause the rising of the valley air. As, however, the increase in dust is 

 frequently accompanied by an increase in the humidity, it seems probable the air came 

 from the valley. Further, there is little or no indication of a daily maximum with winds 

 from unpolluted areas. 



Purifying Areas. 



What may be called purifying areas on the earth's surface are those areas, or districts, 

 in which the atmosphere loses more impurity thau it receives. In all densely inhabited 

 districts it is losing its purity, and in all uninhabited areas it is regaining it, but all 

 uninhabited areas are not equally good purifiers. The quantity of solid matter in the 

 shape of dust and other impurities that are daily thrown into our atmosphere by artificial 

 causes, by volcanic eruptions, and by the disintegration of meteoric matter attracted by 

 the earth is so great, that if it were not got rid of it would soon accumulate and produce 

 conditions very different from those we find at present on the surface of the earth. Part 

 of this dust no doubt settles out and falls to the ground, and can be seen on the surface 

 of snow when it lies a few days, but much of the dust is so fine it will scarcely settle of 

 itself. The deposition of vapour on these very small particles seems to be the method 

 adopted by nature for getting rid of them. As cloud particles form on dust particles, and 

 these in turn form rain drops and fall to the earth, we naturally expect to find that there 

 will be the least dust in the air where this process of purification is going on to the greatest 

 extent. This we find is actually the case. Most of the low numbers in the tables were 

 observed during rainy weather and the very low ones in misty rain, when the clouds were 

 at, or near, the surface of the earth. This experience is confirmed by the observations 

 made on Ben Nevis. At that station, also, the abnormally low numbers were got in cloud 

 and close misty rain ; that is, in the area in which the dust particles are being loaded with 

 water or washed out by the rain. It, however, must be admitted that the evidence of 

 the washing power of the rain is at present not very satisfactory. The air may be 

 purer after the rain, but as we are not then testing the same air we tested before the 

 raiu began, we cannot say what the condition may be of the air which passed us when 

 we made the first test. 



