420 MR JOHN AITKEN ON THE 



by a cold surface, and prevent any possibility of it getting heat after it is expanded, we 

 shall find that all the changes take place very much as if the cold jacket were not there. 



We can further satisfy ourselves in another way, that the clearing is not due mainly 

 to rise of temperature, by varying the experiment in the following way : — Blow steam into 

 the receiver and note the after-effects. In this case, the fog is undoubtedly surrounded by 

 surfaces colder than itself ; and, therefore, any action of these surfaces, either by contact 

 or radiation, is to intensify the fogging. Yet the fogging, as in the previous experiments, 

 rapidly clears. This experiment succeeds best if we use steam under high pressure, 

 issuing from a fine jet, so as to produce a large number of very small drops. 



That the clearing is due to differentiation and falling, is not only proved by the 

 theory already referred to, and by the above experiments, but further confirmation of 

 it may be obtained by examining the cloudy condensation by means of light. We 

 shall see that the colouring produced by means of the small drops, on the light trans- 

 mitted directly through the cloudy air, and also the diffraction colours, are only fine just 

 at the moment of the completion of very rapid expansion. In a few moments the 

 colours begin to fade, showing that the particles are beginning to be of different sizes. If 

 the expansion be made slowly, the colours are never fine, owing to there being time for 

 differentiation to act, and the uniformity produced by quick expansion is never obtained. 

 Differentiation takes place very quickly in the air in these receivers, owing to the particles 

 being very close to each other, and to the very small size of the drops making any slight 

 difference in their size produce a considerable difference in the tension of the water 

 vapour at their surfaces. In clouds, and in all forms of condensation, differentiation goes 

 on much more slowly after the process has been in action for a time. This is due to the 

 drops being now much further apart, owing to the evaporation of the intermediate ones, 

 and also to the fact that when the drops grow to some size any advantage due to differ- 

 ence in curvature falls rapidly. It will be seen from what has been said that fog 

 particles formed on nuclei, which allow the differentiation to take place quickly, do not 

 form thick fogs when the rate of condensation is slow ; and, further, they do not produce 

 fogging unless the air tends to be supersaturated. 



These considerations enable us better to understand the difference between a town and 

 a country fog, and show us why the one is clear compared with the other, setting 

 aside the question of smoke. The country fog, though there may be plenty of nuclei 

 present, is a coarse-grained form of condensation — all the condensing vapour is collected 

 on comparatively few centres ; while in a town fog, the vapour being distributed over an 

 almost infinite number of centres, gives rise to a fine-grained structure, with great light- 

 obstructing powers, and remarkable persistence in duration and elevation. It is thus 

 evidently not so much the number of the dust particles as their composition that we have to 

 fear. Numerous particles of dust, which have no affinity for water vapour, give a dense 

 fogging only when the rate of condensation is very rapid, much more rapid than ever 

 happens in nature ; while particles having an affinity for vapour cause dense fogging 

 under all rates of condensation. 



