SOLID AND LIQUID PARTICLES IN CLOUDS. 315 



700 particles per c.c. The probability seems to be that on this occasion the upper air had 

 about 500 particles per c.c, while the lower air which was rising and forming the clouds may 

 have had somewhere about ten times that number, the ascent of this impure valley air, 

 and its mixture in different proportions with the purer upper air resulting in the forma- 

 tion of masses of air having degrees of impurity varying between 500 and 5000 particles 

 per c.c, as indicated by the numbers in the table. Neither extreme number was, of course, 

 observed, as the conditions would not continue while the whole ten tests were being taken. 



As already stated, the number of particles was always greatest in the clouds. 

 This simply means that the ascending air was both moist and dusty ; and when little of 

 the valley air was mixed with the upper air, the amount of dust was not greatly increased, 

 and the humidity was not sufficient to cause condensation. But when the pro- 

 portion of lower air was large the number of particles was great, and the humidity 

 sufficient to cause condensation. 



It may be mentioned here, that on testing the lower air at the surface of the lake on 

 the afternoon of the same day, the number of .particles was a little under 3000 per 

 c.c It would seem as if the accumulated impurity of the previous day, which had been 

 nearly calm, had risen to the mountain top, and a purer air taken its place, as the 

 maximum observed in the clouds was rather higher than that observed at the level of the 

 lake after the air forming the clouds had risen. 



That there should generally be more solid particles in cumulus clouds than in the air 

 surrounding them, is a result which might have been expected from the conditions. The 

 clouds which form during the day on hill tops are composed mostly of valley air, which 

 has ascended to the upper regions, expanded, cooled, and condensed part of its vapour. 

 The dust in clouds thus acts as a kind of ear-marking, which enables us to trace to 

 its source the air forming the clouds. 



It may be as well to give a word of caution here regarding the observations made 

 of the dust in the clouds and that in the air surrounding them. In all cases there was 

 more dust in the clouds than in the surrounding air. But it by no means follows that 

 this is always the case. It seems quite possible there may be valleys where the air is not 

 polluted, and from which the moist ascending air may be purer than the air at the time at 

 a greater elevation. 



And now a few words as to the water particles in clouds. To those who have been in 

 clouds, especially when they are not dense, and who have felt the glow of heat which 

 radiates from every side, and have seen the surfaces of all exposed objects quite dry, it is 

 difficult to realize that the air is saturated with moisture, and full of suspended drops of 

 water, although those who have thought about the subject may have realized that the 

 thickness is really due to suspended water particles. Yet, so far as I am aware, no one 

 has previously seen, far less attempted to count these drops ; while now, with the aid of 

 the instrument already referred to, these particles can be seen and counted with ease. 



The instrument for observing these water particles consists of a glass micrometer 

 ruled into small squares of 1 mm. or other convenient size. The micrometer is 



