PARTICLES IN FOGS AND CLOUDS. 417 



lower after a time. Probably part of this effect was due to evai3oration, but no doubt 

 very much was caused by the fog, so to speak, raining itself out of existence. 



The above results take place only when the nuclei have no affinity for water vapour, 

 or when there are only a few particles having an affinity amidst a number of neutral 

 ones. If the nuclei have an affinity for water, the differentiating process is interfered 

 with, and the tendency for the condensed water to accumulate on a few drops is checked, 

 and if the supply of moisture is not great it is stopped. Let us suppose that all the dust 

 particles have an affinity for water vapour, produced, say, by burning sulphur, — then, in 

 this case also, the smaller drops will tend to evaporate ; but after they have lost a certain 

 amount of water, the impurities in the reduced drop become more concentrated, and the 

 increasing affinity of the sulphur compound becomes at a certain stage great enough to 

 counterbalance the increased tendency to evaporate due to smallness — that is, the 

 increasing affinity due to concentration becomes at last as strong as the condensing 

 power of the larger drops, the affinity of which has been greatly weakened by the water 

 added to them. The affinity of the nucleus checks the thinning of the fog by differen- 

 tiation and the subsequent falling which would take place if there was no affinity. The 

 affinity of the nucleus thus not only causes condensation to take place before the air is 

 saturated, and causes more centres of condensation to be formed, but it also makes the 

 drops more persistent, and so increases the duration of the fog. 



There are thus, so to speak, both persisting and vanishing fog particles. The former 

 clings the more tenaciously to its vapour the smaller it is ; the latter parts with it the 

 freer the smaller it is. The former tends to produce a maximum number of water 

 particles, with but slight tendency to fall ; the latter a minimum number, with a greater 

 tendency to fall. In the one case there is a struggle for water vapour, while in the 

 other there is a tendency to part with it. The one forms a fog which has a tendency to 

 persist for a long time, while the other forms a fog having a tendency to rain itself away. 



The following experiment may be made in illustration of the contrast between fogs 

 formed in pure country air and those formed in the air of towns, which contains dust 

 particles having an affinity for water vapour. Take two large glass receivers of any 

 form — large flasks have generally been used in such experiments. Provide each flask 

 with a tight-fitting india-rubber stopper, through each of which pass two metal tubes 

 fitted with stopcocks. One of the tubes in each receiver is for introducing the air to 

 be experimented on, whilst the other two tubes are connected together, and they are 

 also connected with another pipe leading to a large metal vacuum receiver, which is also 

 provided with a stopcock. By this arrangement we can introduce into the two receivers 

 airs containing the different kinds of nuclei we wish to experiment upon, and when the 

 stopcocks connecting the receivers with the metal vacuum receiver j are opened, the 

 contents of the two glass receivers are subjected to the same expansion and cooling. The 

 two volumes of air are thus under the same conditions, except as regards the difference 

 in nuclei which we have introduced for the experiment. 



At the beginning of the experiment the pressure in the metal vacuum receiver is 



