178 Proceedings of the Royal Society of Edinburgh. [Sess. 
form clouds unless it has some solid or liquid nucleus to condense upon.* 
Dr Aitken worked out this whole research with unswerving zeal, clearing 
away by a magnificent series of control experiments many objections which 
seemed at first sight difficult to meet and even inconsistent with the broad 
theory. He brought into prominence the vast importance of the dust in 
the atmosphere, not only visible dust but the impalpable dust particles 
which provide nuclei for the condensation of vapour and the formation of 
visible drops of rain or mist. By an interesting process of evolution he 
gradually constructed a form of apparatus by which, from the number of 
raindrops produced in a closed region of saturated air, he was able to 
calculate the number of dust particles in this region. A slight expansion 
by means of an air-pump in connection with the closed region produced 
a cooling in the saturated air, from which the vapour condensed on the 
dust particles and formed tiny drops of water. These, falling on a silvered 
surface ruled in small squares, were readily counted. This was the so-called 
Dust-counter, the final portable form of which was an instrument of con- 
siderable precision in the hands of the skilful meteorologist. 
The production of a fog cloud in a receiver from which saturated air 
was being extracted was a phenomenon which had often been seen by 
experimenters; but it was reserved for John Aitken not only to give a 
complete explanation of the phenomenon but to open up an entirely new 
line of research. 
Aitken s experiments proved that when the saturated air was free of 
dust no cloudy condensation took place on slight expansion, for there were 
no particles to serve as nuclei. He found, however, that once the air was 
cleared of dust by filtration through cotton- wool, a more rapid expansion 
sometimes led to cloudy condensation. The explanation of this was 
subsequently given by C. T. R. Wilson, who showed that ionised air, 
although dust free, produced cloudy condensation when a considerable 
expansion with accompanying cooling took place. There has consequently 
been a tendency in some quarters to explain condensation of vapour in 
terms of the presence of ions, arguing that Aitken’s dust particles were 
unnecessary as a factor in the process. But such a view shows an absolute 
lack of appreciation of the whole meaning of the phenomenon. The sudden 
expansion and cooling required to produce cloudy condensation on ions 
are much greater than can ever occur in nature. On the other hand, when 
dust particles are present a very slight expansion with accompanying slight 
* See “On Dust, Fogs, and Clouds,” Trans. Roy. Soc. Edin., xxx, 1880-1 ; and various 
papers on dust particles in the air, Trans. Roy. Soc. Eclin., vols. xxxv to xxxix, 1887-1899 ; 
and many papers in the Proceedings. 
