1889-90.] 
Mr J. Aitken on Dust Particles. 
211 
and widely separated, so that each particle would be large and fall 
rapidly as rain. At present therefore all the evidence points to the 
conclusion that dust is present in our atmosphere at whatever eleva- 
tion clouds form. 
Now though this conclusion maybe true, so that on the one hand 
we may have evidence that at very great heights in our atmosphere 
there may be enough dust to form clouds, and that while experi- 
ment shows there may be only 200 particles per c.c. in the air, w T e 
are yet without any guide as to what is the number of particles in 
pure unpolluted air. The air at tops of mountains, and even at great 
heights away from mountains, may be polluted by artificial causes. 
The dust of the lower air will be carried by ascending currents to 
great heights, and much that is present in what we have called pure 
air may be of terrestrial origin. But even supposing the lower air 
never carried dust to great heights, yet there will be dust there, due 
to the disintegration of the vast quantities of meteoric matter, which 
is daily showered down into our atmosphere, where it becomes 
highly heated and dissipated, thus forming fine meteoric dust. 
What the number of particles of this meteoric dust may be, we 
cannot say, nor can we tell what its proportion to terrestrial dust 
may be at the different elevations, but it seems probable that the 
meteoric dust will vary in quantity from time to time, with the 
earth’s position in space. 
Dust and Atmospheric Transparency. 
When I made my last communication to the Society on atmo- 
spheric dust, * reference was made to the close relation which was 
observed between the transparency of the air and the number of 
particles of dust in it, when the tests were made at Colmonell. At 
the same time it was pointed out that this relation was likely to 
hold good only under certain conditions, that the amount of vapour 
in the air, or rather the humidity or degree of saturation, would be 
likely to modify the effect of the dust. My reason for qualifying 
the Colmonell conclusion was that it had been shown in a previous 
paper f that there is an affinity between the dust in our atmosphere 
and water vapour, and that some kinds of dust begin to condense 
I * Proc. Roy. Soc. Edin ., vol. xvi. p. 160. 
t Trans. Roy. Soc. Edin., vol. xxx. part i. 
