16 Mil JOHN AITKEN ON 



The principal object of the two papers by Helmholtz and Richarz is to prove that 

 cloudy condensation may take place without dust, and the authors describe a great number 

 "1 very beautiful experiments made with the steam jet, showing the change which takes 

 place in its appearance when mixed with air charged with the products of combustion 

 either from flames or from slow combustion, such as damp phosphorus slowly oxidising in 

 the aii', or when the jet is mixed with air which had been exposed to the electric discharge. 

 In all these cases the authors show that something is produced which, when it meets 

 the steam jet, causes an increase in the density of the condensation, evidently due to 

 an increase in the number of small water drops. That there is something produced 

 which causes an increase in the density of the steam is evident ; but whilst one may 

 go thus far with these authors, yet it is difficult to accept their explanation of the 

 phenomenon. They attribute the condensation not to nuclei, but to the molecular 

 shock produced by the chemical processes going on in the neighbourhood of the jet, 

 or, to use a translation of the authors' words, "The collapse of the unstable condition 

 of the supersaturated water vapour from the jet is determined less by the particular 

 nature of the chemical action than to a far greater extent by the molecular shocks set 

 up in the jet, molecular dissociations and associations, and on the presence of unsatu- 

 rated compounds, molecular groups with free valencies or ' ions.' " 



Whilst accepting the facts of the experiments, namely, that something is produced 

 in these chemical processes which determines dense condensation in the steam jet, yet 

 one has great difficulty in accepting the above explanation. Supposing we even admit 

 that a molecular shock of the kind described could determine condensation in a super- 

 saturated vapour, it must be remembered that the degree of supersaturation in a 

 .steam jet in the open air where there is dust is extremely slight, — the particles of 

 water are so close that any strain easily relieves itself. Further, it must be remem- 

 bered that the vapour in the jet is nearly in equilibrium with the drops of the size 

 present in the jet, and it is very doubtful if it will be at all supersaturated to extremely 

 minute particles of water, as the vapour tension at the surface of extremely small 

 particles is higher than at the surface of larger particles. For these reasons it seems 

 more probable that whatever it is that is produced in these chemical processes, 

 it acts somehow in forming nuclei, and is itself engaged directly in the formation 

 of the centres of condensation.* 



Near the end of the second paper already referred to, the authors put in a claim 

 for the importance of these " ions " in producing ordinary cloudy condensation. They 



* There is another way of considering the whole question, from which it would appear that there is no such 

 thing as aupereaturation in a vapour; that is, no strain in the vapour which either dust or "molecular shocks" 

 can relieve. What is generally called a saturated vapour is one whose tension is equal to the tension of the vapour 

 at a flat surface. Now this tension is not so high as the tension at a surface of extremely small convex curvature ; 

 and vapour that is in equilibrium with the vapour at a convex surface is supersaturated to the flat surface ; so that 

 saturation is a relative and not an absolute quantity, relative to the curvature of the condensing surface, and a 

 vapour that is supersaturated to a Hat surface is not necessarily saturated to a surface of very small curvature. It 

 would thus appear that there is no strain in a vapour till a surface makes its appearance ; but after it is formed 

 the lower tension at its surface determines a movement of the vapour molecules towards it. 



