delighted audiences here with an exposition of
the effect of the scattering of light by small particles in the formation
of artificial skies, and it would be superfluous for me to enter more
into that. Suffice it to say that when particles are small enough to form
the artificial blue sky, they are fully small enough to obey the above
law, and that even larger particles will suffice. We may sum up by saying
that very fine particles scatter more blue light than red light, and that
consequently more red light than blue light passes through a turbid
medium, and that the rays obey the law prescribed by theory.

I will exemplify this once more by using the whole spectrum and placing
this cell, which contains hyposulphite of soda in solution in water, in
front of the slit. By dropping in hydrochloric acid, the sulphur
separates out in minute particles; and you will see that, as the
particles increase in number, the violet, blue, green, and yellow
disappear one by one and only red is left, and finally the red disappears
itself.

Now let me revert to the question why the sun is red at sunset. Those who
are lovers of landscape will have often seen on some bright summer's day
that the most beautiful effects are those in which the distance is almost
of a match to the sky. Distant hills, which when viewed close to are
green or brown, when seen some five or ten miles away appear of a
delicate and delicious, almost of a cobalt, blue color. Now, what is the
cause of this change in color? It is simply that we have a sky formed
between us and the distant ranges, the mere outline of which looms
through it. The shadows are softened so as almost to leave no trace, and
we have what artists call an atmospheric effect. If we go into another
climate, such as Egypt or among the high Alps, we usually lose this
effect. Distant mountains stand out crisp with black shadows, and the
want of atmosphere is much felt. [Photographs showing these differences
were shown.] Let us ask to what this is due. In such climates as England
there is always a certain amount of moisture present in the atmosphere,
and this moisture may be present as very minute particles of water--so
minute indeed that they will sink down in an atmosphere of normal
density--or as vapor. When present as vapor the air is much more
transparent, and it is a common expression to use, that when distant
hills look "so close" rain may be expected shortly to follow, since the
water is present