226 Proceedings of Royal Society of Edinburgh. [sess. 
gradual thickening has been frequently seen. At the Italian lakes, 
on many occasions, when the air was damp and still, there has been 
noticed in close proximity every stage of condensation, not divided 
by the hard line usually observed, but where it was impossible to 
say where the thick air ended and the cloudy began. Again, this 
gradual change can often be seen in the sky overhead. With 
approaching change of weather, the sky is often seen to change by 
imperceptible degrees from perfect transparency to “thick,” and 
then to cloud. It is very doubtful if we are entitled to expect to 
find any well defined haze in the air immediately surrounding 
clouds, as we do not know sufficient about the conditions existing 
there. It may be as well to note here that on all occasions on 
which I have had an opportunity of making observations on this 
point, that the transition from moist air in the cloud to dry air 
immediately outside it was very rapid, so rapid as to give only a 
sharp outline. 
Haze. 
The conclusion to which these observations point is that haze is 
caused by the dust in our atmosphere, and that this dust has in 
almost all degrees of humidity more or less water attached to it. A 
thick haze may be the result of much dust and little moisture or of 
little dust and much moisture, but the moisture less than saturation. 
According to this view, in most conditions of our atmosphere, haze 
is but an attenuated or arrested form of condensation — arrested for 
want of moisture, or it may be a decayed form of condensation, that 
is, cloudy condensation changed to haze by the reduction of its 
humidity, possibly by rise of temperature, and the fog particles 
evaporated, till they hold only the water of attraction. If the 
humidity be decreased still further the haze clears more and more, 
but the dry dust still has a hazy or thickening effect on the 
atmosphere. 
This conclusion is confirmed by an examination of the figures in 
the table, where it will be seen that whenever the air was dry 
and hazed there was much dust in it, and that as the dust 
decreased the haze likewise decreased. It is well known that hot 
weather is often accompanied by a thick haze. The explanation of 
this would appear to be, that during hot weather we have generally 
much dust in the air, and further as has been already explained 
