October 6, 1923] 



NA TURE 



495 



Aitken's Scientific Papers. 



Collected Scientific Papers of John Aitken, LL.D., F.R.S. 

 Edited for the Royal Society of Edinburgh (with 

 Introductory Memoir) by Dr. C. G. Knott. Pp. 

 xxi + 591. (Cambridge: At the University Press, 

 1923.) 30s. net. 



^HE late Dr. John Aitken bequeathed to the Royal 

 Society of Edinburgh a sum of loooZ. to be 

 expended in issuing a reprint of his more important 

 scientific papers. The work of editing the collection 

 was assigned by the Society to its General Secretary, 

 the late Dr. C. G. Knott, and the present volume is the 

 result. 



Aitken's contributions to science and to its literature 

 extend over half a century, and include about a hundred 

 i papers contributed to various societies and periodicals. 

 [The subjects ranged over a remarkably wide field. 

 [Safety-valves on steam boilers, colour vision, glaciers, 

 Ithermometer screens, colours in the sky and sea, are 

 [only a few of the subjects dealt with beyond the main 

 fwork which occupied his attention for more than forty 

 fyears. The selection here presented consists of thirty- 

 eight of the more important papers, and has been made 

 [with great care. The volume, which includes a brief 

 t account of Aitken's life and work, meets a real need, 

 (for in recent years the Royal Society of Edinburgh has 

 had to reprint some of Aitken's papers more than once. 

 The most notable contribution made to science by 

 [ Aitken was his study of dust in the atmosphere and of 

 I the physical phenomena to which it gives rise. This 

 forms the subject of no fewer than fourteen of the 

 collected papers. He was drawn to the inquiry from 

 consideration of the phenomena accompanying changes 

 of state and especially of the acceleration of such 

 \ changes in the presence of " free surfaces." In his first 

 paper, on " Dust, Fog, and Clouds," he states his main 

 conclusion. 



" Molecules of vapour do not combine with each other 

 and form a particle of fog or mist ; but a free surface 

 must be present for them to condense upon. The 

 vapour accordingly condenses on the dust suspended in 

 the air, because the dust particles form free surfaces 

 at which the condensation can take place at a higher 

 temperature than when they are not present. Where 

 there is abundance of dust there is abundance of free 

 surfaces, and the visible condensed vapour forms a 

 dense cloud : but when there are no dust particles 

 present there are no free surfaces, and no vapour is 

 condensed into its visible form, but remains in a 

 supersaturated vaporous condition till the circulation 

 brings it into contact with the free surfaces of the sides 

 of the receiver, where it is condensed." 



Aitken was not the first to reach this conclusion, for 

 he had been anticipated by Coulier, whose results had 



NO. 2814, VOL. I 12] 



been published five years earlier. But of the absolute 

 independence of Aitken's work there can be no doubt, 

 and his more extensive researches opened out the field 

 of inquiry in such a manner that his name will always 

 be associated with the subject. He proceeded in later 

 papers to develop it further, to describe ingenious 

 apparatus devised for counting the number of dust 

 particles (or rather, condensation nuclei) in unit volume 

 of air, and to state the results of a large number of 

 observations, in widely different conditions, on the 

 " dustiness " of air in houses, towns, open country, and 

 seashore, and on mountain heights. 



The present writer heard Aitken giving some of these 

 papers to the Royal Society of Edinburgh, and had the 

 privilege of working at the subject under his guidance. 

 One was at first struck with the confidence with which 

 Aitken stated his results, but there was always the note 

 of reservation when a possible alternative was presented. 

 Looking back on these early days, and in the light of 

 later work, it can be seen that although his results 

 seemed straightforward and their interpretation ob- 

 vious, Aitken was troubled by the fear that something 

 more lay behind them. This is evident from his 

 guarded language in speaking of the arrangements for 

 filtering dust out of a sample of air and his insistence on 

 the readiness with which condensation takes place in 

 the presence of alkaline salts and sulphur compounds. 

 The fuller knowledge came later with Wilson's experi- 

 ments on the condensation of supersaturated vapour 

 upon the ions in a gas ; and evidence, collected together 

 in Dr. Simpson's recent Royal Institution lecture 

 (Nature Supplement, April 14), has accumulated to 

 show that condensation at or near normal pressure 

 takes place only on the hygroscopic dust particles. In 

 another direction, too, Aitken's work has been supple- 

 mented. His explanation of the production of fog, 

 especially the smoky fog of towns, was insufficient inas- 

 much as it (necessarily at that time) took no account of 

 those temperature inversions at comparatively low 

 altitudes which prevent the lateral or vertical escape 

 of smoke-laden foggy air. But he was more nearly 

 correct in his deduction that — 



" We must remind those who are crying for more 

 perfect combustion in our furnaces and grates that 

 combustion, however perfect, will not remove or 

 diminish fogs. It will, however, make them cleaner, 

 take away their pea-soupy character, but will not 

 make them less frequent, less sulphurous, less per- 

 sistent, or less dense." 



Aitken's next contribution of importance was his 

 paper " On Dew " (1885), in which he showed that 

 deposits of dew arc produced by the condensation of 

 water vapour rising from the soil, and that the dewdrops 

 on grass are formed from water exuded from the pores 



