TBANSACTIONS. 



I. — On the Number of Dust Particles in the Atmosphere. By John Aitken, Esq. 



(With a Plate.) 



(Read 6th February 1888. 

 ' The gay motes that people the sunbeams. " — Milton. 



The solid matter floating in our atmosphere is every day becoming of greater and 

 greater interest as we are gradually realising the important part it plays in the economy 

 of nature, whether viewed as to its physical, physiological, or meteorological aspects. 

 One fundamental point on which we have at present very little information of anything 

 like a definite character, is as to the number of solid particles present in our atmosphere. 

 We know that they are very numerous, and it seems probable that the number varies 

 under different conditions of weather ; but what number of particles are really present 

 under any conditions, and how the number varies, we have at present very little idea- 

 In this field of research the physiologists are far in advance of the physicists, as they have 

 devised means of counting the number of live germs floating in our atmosphere, and 

 already we have a good deal of information as to how the number varies under different 

 conditions. 



Part of our ignorance regarding the dead organic and inorganic matter floating in the 

 atmosphere may no doubt be due to the apparent difficulty of an investigation of this 

 kind, where many of the particles to be counted are so extremely small that they are not 

 only invisible to the naked eye, but are beyond the highest powers of the microscope. 

 But, though this may be the case, yet the real reason of our ignorance on this subject 

 would rather appear to be, that no one has seriously set himself to the investigation of the 

 subject ; the problem, though apparently one of considerable difficulty, has turned out 

 to be of comparatively simple solution, when the degree of accuracy required is not very 

 great. When we consider the numbers we have to deal with, and the conditions of the 

 problem, perfect accuracy does not indeed seem possible. 



VOL. XXXV. PART 1 . A 



