1889 - 90 .] 
Mr J. Aitken on Dust Particles. 
233 
fair number of particles, and only decreased after blowing some 
time. 
When on the Eigi it was also noticed when there were clouds 
in the valley, that the wind in driving them along, also drove them 
up the slopes of the mountains, sometimes to the very top. The 
condensation of the moist air in the clouds supplied some of the energy 
necessary for the ascent, as in some cases the clouds were seen to leave 
the hill-side and rise nearly vertically. We therefore seem to have 
very good reason for supposing that even at such an isolated position 
as the Eigi Kulm the air is more or less polluted with valley air. At 
stations on the tops, or on the slopes of long ridge-shaped hills, the 
air will be practically valley air when the wind is at right angles to 
the ridge. 
There is another point which I noticed at this elevated situation, 
with which I shall close these Eigi notes. It has reference to the 
colouring of earth and sky seen at sunrise and sunset when observed 
from the top of mountains. I think the general impression is that 
the colouring is much finer on these occasions when seen from an 
elevation than from the valley. How the result of my observations 
during my stay at the Eigi Kulm all point the other way. 
The weather was favourable for seeing the sun. rise and set on 
all the days I was there, and yet on none of them did I see any 
display of colour ; indeed, I was much struck by the want of it. 
On the 24th, which was the finest sunrise during the time, the dis- 
tant snows were tinged slightly red for a few minutes only while the 
sun -was just on the horizon; immediately it got a very little clear of 
the horizon all colour was gone. The same was the case with the 
sunsets, greys predominated over other colours. How during this 
time I was told that, as seen from Lucerne, the sunsets were remark- 
ably fine for colour effects. This would seem to indicate that the 
colouring, at least under the conditions existing during my visit, 
was mostly done by the lower air — that the pure air between the 
mountains and the sun robbed the sun’s rays of but little of their 
blue light, and that it was the lower impure air between the hills 
and the observer to which most of the colouring was due. 
This supposition, that the lower dusty humid air is the chief 
cause of the colour in sunset effects is supported by other observa- 
tions. When on the top of the mountain I frequently saw large 
