230 THE BIOLOGY OF THE SEASONS 



and dried, leaving a circular mark with a dust particle in 

 the centre. 



Mist, fog, cloud, and rain thus consist of more or less 

 wet dust-particles of great fineness ; or, to say the same 

 thing once again, fog is dust-dew. Familiar facts now, 

 but surely not without a certain poetry. 



Aitken was led to a clever contrivance for actually 

 counting the myriad atomies of dust which play so im- 

 portant a part in making that curious mixture which we 

 call weather. The device was to make rain in the air, and 

 then count the drops. A measured sample of the sur- 

 rounding atmosphere is drawn into a receiver, where it is 

 diluted with a certain quantity of dustless air, and then 

 saturated with water. By a stroke or two of an air-pump, 

 it is expanded and chilled till the water vapour and the 

 dust fall together in a mimic shower of raindrops on a 

 polished plate. 



When the miniature shower is over, that is to say, when 

 all the dust has fallen, the number of drops on a measured 

 area of this plate is carefully observed, and as each drop 

 has a dust particle in its heart, it is possible in this way to 

 count the motes which seem so incalculably numerous as 

 they " dance " on the path of a " beam of light." 



By this simple apparatus for making rain and counting 

 motes, Dr. Aitken was able to gauge the quality of the air 

 in all sorts of places and conditions, and his method or some 

 modification of it is now in everyday use in Public Health 

 Departments and the like. We must be content to give 

 a few illustrative instances of this sampling. In the lecture- 

 room of the Royal Society of Edinburgh, before one of the 

 meetings, the air near the floor showed 275,000 dust- 

 particles in a cubic centimetre, but after the lapse of an 

 hour and three-quarters of meeting the scientific atmo- 

 sphere showed 400,000. Of obvious importance in connec- 



