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Mr. J. Aitken. On some Phenomena [Apr. 28, 



V. " On Some Phenomena connected with Cloudy Condensa- 

 tion." By John Aitken, F.R.S. Received March 2, 

 1892. 



In the first part of this communication I intend giving the results 

 of an investigation into the phenomena connected with the cloudy 

 condensation produced when a jet of steam mixes with ordinary air, 

 with special reference to the marked change which takes place in the 

 appearance of the jet by electrification and other causes. In the 

 second part will be given the results of an investigation into certain 

 colour phenomena which can be produced when the condensation is 

 made to take place under certain conditions, and it is thought that 

 these experimental colour phenomena, if they do not give the ex- 

 planation of a " green sun," at least enable us to reproduce it arti- 

 ficially with the materials existing in our atmosphere. 



Paet I. — Steam Jets. 



When a jet of steam escapes into the air, condensation at once 

 ensues by the expansion and the mixing of the steam with the cold 

 air. The result is the jet becomes distinctly visible by the light 

 reflected by the minute drops of water carried along in the mixed 

 gases and vapour. At first sight there is little that is interesting in 

 the changes then taking place. The subject has, therefore, attracted 

 but little attention, and has been but little studied. This is evident 

 from the great interest that has been taken in the change produced 

 in the appearance of the jet when it is electrified ; yet I hope to be 

 able to show that this is only one of a number of causes which alter 

 the appearance of the condensing steam. 



R. Helmholtz was the first to show that when an ordinary jet of 

 steam is electrified, there is a marked increase in the density of 

 the condensation. The effect of the electricity is certainly very 

 remarkable. The instant the jet is electrified, it at once changes 

 and becomes much denser, and the condensed particles also become 

 visible much closer up to the nozzle from which the steam is escaping. 

 For the convenience of description we shall call this second form of 

 condensation dense condensation, while that usually observed we shall 

 call ordinary condensation. Not that there is any hard and fast line 

 between these two forms, as the one may be made to change by 

 imperceptible degrees into the other. All that is meant is that the 

 one is dense compared with the other. 



One result of this investigation is that, in addition to electrification 

 of the jet, there are four other ways in which the ordinary condensa- 



