414 MR JOHN AITKEN ON THE 



revealed itself. On watching the micrometer, as its temperature was gradually lowered 

 by the snow, there was seen, just a moment before the surface became dewed, a vast 

 number of drops, so small that they were only just visible, and so numerous that it was 

 quite impossible to count the number falling on one square millimetre. The reason the 

 drops were not seen at first was their very small size ; and either the focussing was not 

 good enough, or the very small particles evaporated in the slightly heated air over the 

 micrometer, or they evaporated so quickly on touching it, that the eye was incapable of 

 detecting them. But by cooling the micrometer to its dew-point they touched the glass, 

 and remained long enough to be seen before they vanished. If the micrometer had been 

 kept at the correct temperature, they might have been observed for some time; but owing 

 to the cooling produced by the snow, the micrometer was cooled below the dew-point, 

 after which the drops were again invisible, as they fell on a wet surface. It was only 

 when the micrometer was a fraction of a degree above the dew-point that these very 

 small particles were seen, and they were observed to fall very irregularly owing to the 

 amount of wind. At one moment none were seen ; the next they were so numerous that 

 they seemed to cover the whole surface. The number of drops falling on this occasion 

 would therefore seem to have been very much greater than previous observations would 

 have led us to expect. 



One naturally asks, why this difference in the number and size of the drops on this 

 occasion ? In all the clouds previously observed, the drops were of a fair size and 

 easily seen with the power used in the fog-particle counter, and the size did not seem to 

 vary much on the different occasions. During the previous winter I had been engaged 

 in some experiments on cloudy condensation. The results of these experiments were 

 communicated to the Eoyal Society.* In that communication it is shown that the 

 number of dust particles which become active centres of condensation — that is, the 

 number of water particles in cloudy condensation — depends on the rate at which the 

 condensation is made to take place. The quicker the condensation the greater the 

 number of dust particles forced to become centres of condensation, and the slower the 

 rate the fewer the drops produced. It is further shown, that after the rate of con- 

 densation becomes slower, or after it has ceased altogether, a process of differentiation 

 amongst the drops begins. Some of the drops increase in size, whilst others decrease 

 and get dried up, so that a cloudy condensation, which, when first formed, was dense 

 and full of water particles, soon loses its denseness by the diminution in the number 

 of its drops. 



When I had the peculiar experience on the Rigi Kulm, on the morning of the 21st 

 of May, I was therefore in a manner prepared with an explanation of the exceptionally 

 large number of very small drops. The same thing had been seen in another form, 

 over and over again, in the laboratory, but this was my first experience of it in nature. 

 In all the clouds previously tested the condensation would seem to have been formed 

 slowly, or the observations were made in old clouds, after the particles had undergone 



* Roy. Soc. Proc, vol. li. p. 408. 



