422 



Mr. J. Aitken. On some Phenomena [Apr. 28, 



the cooling has any influence. The same explanation holds good for 

 the limited range of the action of obstructions in front of the jet. 



At a distance of a few centimetres from the nozzle new drops seem 

 still to be forming, as the density of the condensation is slightly 

 increased by increasing the supply of nuclei at that distance. The 

 drops seem also occasionally to coalesce at a distance of 3 or 4 cm. 

 from the nozzle, as electricity slightly increases the density of the 

 condensation even at that distance. 



Part II. 



Colour Phenomena connected with Cloudy Condensation. 



In the following remarks it is not intended to discuss the many 

 colour phenomena which are known to be connected with cloudy con- 

 densation. Attention will be confined principally to some new 

 phenomena, the experimental illustration of which has been developed 

 in the present investigation. 



Before describing these experiments, it may be as well to 

 refer to some changes which take place in the constitution of 

 cloudy condensation, both while it is forming and after it has been 

 developed, as it will be necessary for us to keep certain points in view 

 while discussing the colour phenomena. There are two points to 

 which special reference is required. These are, first, the manner in 

 which the appearance of the condensation is affected by the greater 

 or less degree of supersaturation, that is, by the rate at which the 

 condensation is made to take place ; and, second, the changes which 

 take place in the appearance of this cloudy condensation after the 

 tendency for the vapour to deposit has stopped. 



These two points may be best discussed by taking the second 

 first. Suppose we blow some steam into the air inside a glass vessel, 

 and leave it undisturbed ; if we examine the cloudy condensation 

 after a time, we shall find that a considerable change has taken place 

 in its appearance. The change is due to two causes : part is due to 

 the gradual descent of the particles by which a clear space is formed 

 in the upper part of the vessel. But it will also be observed that the 

 clouding in the lower part is much thinner than it was at first. 

 Probably part of this thinning is due to some of the particles having 

 fallen to the bottom of the vessel, but this is not the principal cause 

 of the change. The thinning is due mainly to a reduction in the 

 number of particles in the air, by the smaller particles gradually 

 becoming absorbed by the larger ones. This is caused by the vapour- 

 pressure at the surface of small particles being greater than at the 

 surface of larger ones, with the result that the smaller particles 

 evaporate in air of the same humidity in which the larger ones are 

 condensing vapour. 



