368 JOHN AITKEN ON DUST, FOGS, AND CLOUDS. 
the quickly falling particles with those falling more slowly. It would thus 
seem that while on one side if we have no dust we would have no clouds and 
probably no rain, as we don’t know whether the air would ever become suffi- 
ciently supersaturated to condense without nuclei. On the other hand, an 
over-abundance of dust having affinities for water vapour also prevents the 
vapour falling as rain, as the vapour under these conditions condenses into 
minute particles which all tend to be of equal size, and none of them are able 
to fall quickly enough amongst the others to cause collisions. The result is the 
condensed vapour cloud instead of falling in minute parts as rain, tends to fall 
as a whole. The air becomes so loaded with the water held in mechanical 
suspension that it is dragged downwards by its weight. If we make artificial 
fogs with sulphur fumes and ammonia, or by heating chloride of magnesium, 
the fog is so heavy it can be poured from one vessel to another. 
After the affinities of the dust particles are satisfied, this tendency to - 
stability no longer exists. After this stage the growth of the particles becomes 
unequal, and, as has been shown by Professor CLERK MAXWELL,* the larger 
drops or particles in a cloud tend to rob the smaller ones, or rather, from what 
we now know, will tend to prevent them growing after the affinities of the 
nuclei are satisfied. 
It would appear, then, that condensation will always begin in our atmosphere 
before the air is saturated. There is, however, still much to be done in this 
department of our subject to determine whether the amount of cloudy conden- 
sation is always the same for the same degree of saturation, or if it varies; and 
if it varies, to find the composition and source of the nuclei which cause the 
variations. 
I feel that these two papers only start this inquiry. Much, very much, still 
remains to be done. Like a traveller who has landed in an unknown country, 
I am conscious my faltering steps have extended but little beyond the starting- 
point. All around extends the unknown, and the distance is closed in by many 
an Alpine peak, whose slopes will require more vigorous steps than mine to 
surmount. It is with reluctance I am compelled for the present to abandon 
the investigation. It is, however, to be hoped it will be taken up by those 
better fitted for the work, and that soon the roughness of the way will be 
levelled, the difficulties bridged, the country mapped, and its resources 
developed. 
* «Theory of Heat,” Professor CLERK MaxwE.L, p. 270, 
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