PARTICLES IN FOGS AND CLOUDS. 415 



the process of differentiation, whereas on this occasion it seemed probable that the 

 observations were made in a new cloud not only in the process of condensation, but 

 also in the process of very rapid formation, as the wind was blowing with considerable 

 force, and the air in the cloud was rapidly expanded, owing to the great velocity with 

 which it was forced up the mountain slopes. Further, the air was coming from the 

 direction of Lucerne, and was very impure, some of the tests showing as many as 7000 

 dust particles per cubic centimetre. Now, these are the very conditions which, we know 

 from experiment, give rise to a large number of small drops in cloudy condensation. 



At the time, however, I was not quite satisfied with the above explanation, as when 

 these very small drops were observed there was also a slight fine rain. Now, if the cloud 

 were new, and in the very process of formation, and if no differentiation had taken place, 

 how were we to explain the presence of the fine rain ? As, however, we know so little about 

 what determines the formation of rain-drops, it might be as easy for us to suppose it may 

 take place in newly-formed as well as in old clouds. It was not, however, necessary to 

 assume this, as time, patience, and observation got over the difficulty. Breaks occurred 

 from time to time in the clouds on the mountain, and at last they ceased forming 

 altogether. But though these clouds were gone, the fine rain continued, and a higher 

 stratum of cloud became visible. It therefore seemed extremely probable that it was from 

 this higher stratum that the rain had been falling which was observed in the cloud on 

 the mountain, and that it had not been formed in the newly-made lower cloud. 



It would therefore appear from these observations on the Rigi Kulm, that we must 

 modify our conclusions regarding the relation between the density or thickness of a 

 cloud and the number of water particles in it. This relation would appear to exist only 

 when the clouds compared are at a corresponding stage of their development. But from 

 the fact that differentiation takes place very slowly after the cloud has been in exist- 

 ence for some time, owing to the drops having become fewer and further apart, thus 

 greatly decreasing the rate at which the vapour exchanges can take place, and to the 

 slight difference in condensing power in the different drops when they have grown to 

 some size, it seems probable that the relation referred to may exist in all clouds which 

 have been formed for some time. These thoughts point to the conclusion that cloud 

 particles, like most things in this world, have a birth, development, and decay, and that 

 on these the life of the cloud greatly depends. 



Fog Particles. 

 In the former papers on this subject I have dealt principally with the formation of 

 fogs. On the present occasion attention will be principally directed to the persistence 

 of fogs after they have been formed, to what might be called the duration of their life. 

 The remarks to be made are not in themselves of much value ; but as the subject is one 

 of very general interest, and of great importance from a sanitary point of view, every 

 little addition to our knowledge may be valuable by reason of the assistance it may give 

 to others working in the same field. 



