1920-21.] Obituary Notices. 179 
cooling suffices. An experiment often made is to hold a bunsen flame for 
a moment within a receiver, set the receiver immediately on the air-pump 
plate with a dish of water within it, and then pump some of the air out. 
A dense fog cloud is formed, and this is not unfrequently referred simply 
to the ionisation due to the flame. But the argument is faulty, for of 
course there are numerous dust particles also produced by the flame, and 
it is impossible in such an experiment to discriminate between the effect 
of the particles as fog producers and the effect of the ions. Moreover, 
Aitken himself proved that when dust particles were undoubtedly present 
electrification of the air did not increase the cloudy condensation. 
When we recognise that dust particles are always present in the 
atmosphere, and that a slight cooling of the saturated air is the cause of 
the production of raindrops, and when we further bear in mind the 
beautiful demonstration given by Aitken that no cloudy condensation is 
produced in saturated dustless air on slight cooling, there is no escape from 
the conclusion that mist, fog, and cloud require for their formation the 
presence of dust particles. 
Another important direct result of Aitken’s experiments on cloudy 
condensation, and especially of his methods of counting the raindrops 
formed, is worthy of mention. Sir J. J. Thomson in his classical experi- 
ments on the mass and charge of an electron made use of Aitken’s method 
of condensation in obtaining one of the measurements on which the deter- 
mination of these two small quantities depended. 
Meanwhile, Aitken himself pushed his own investigations in many 
directions, such as the meteorological and industrial conditions governing the 
production of dust particles in the air, the influence of locality and altitude, 
the effect of prevalent winds and of cyclonic and anticyclonic distributions. 
Closely connected with this whole research is his important paper on the 
formation of dew.* His views, though now generally accepted, were strongly 
combated by certain authorities at the time of their first promulgation. 
What he showed by skilfully arranged experiments was that the vapour 
which condenses as dew on cold surfaces comes mainly, if not entirely, from 
the ground below and not from the air above. He also showed that the 
so-called dewdrop on leaves of plants was not dew at all, but was exuded 
sap. He has also placed on record some interesting observations on hoar 
frost; and in a paper published in the Journal of the Scottish Meteorological 
Society he has given a remarkably clear description of the formation of 
ground ice. 
In his presentation of papers before our Society, in whose Transactions 
* See “ On Dew,” Trans. Roy. Soc. Edin ., xxxiii, 1885. 
