24 8 
On Water and Air . 
[April 
the power of the visible spectrum ; so that you have invi- 
sible rays equal to between seven and eight times the visible 
in point of energy. 
In our last ledture I tried in your presence to separate 
one class of rays from the other. Here is a very beautiful 
clear liquid, called bisulphide of carbon. Here is an element 
called iodine. You see it here in the form of crystals. If 
I pour some bisulphide of carbon into this glass cell, and 
drop into it some of our crystals of iodine, you will see that 
the liquid instantly becomes dark. This is the solution 
which I used in our last ledture for the purpose of cutting 
off the visible rays and allowing the invisible rays to pass 
through. The sharpness with which that solution cuts off 
the light of the spedtrum, and allows all that mountain of 
heat, as I have expressed it, to pass through, is amazing. 
As I have said, this is the preparation which was used to 
sift or filter the radiation which was given forth by our little 
sun, and to enable us to see what could be done by the invi- 
sible rays alone. 
Well, as I have said, these invisible rays are those which 
are instrumental in raising the vapour of the ocean into 
the air and into the region of the clouds. But how is it 
that this vapour, which on starting is perfedtly invisible, 
becomes cloudy ? That you will understand immediately. 
You saw in our last ledture that, by the application of heat, 
we were able to convert a visible cloud into invisible vapour ; 
and now I want to show you the effedt of the simple 
expansion of air. The air here, at the lower portions of our 
atmosphere, has to bear the weight of all the atmosphere 
above it. The consequence is, that it is compressed or 
closely compadted together ; and when that air ascends, 
carrying with it a load of aqueous vapour into the upper 
regions of the atmosphere, the pressure upon it becomes 
less and less, and the consequence of this is that it expands 
more and more, and by its expansion it chills itself. The 
chilling thus produced is competent to cause the invisible 
vapour which started from the ocean to curdle up as cloud. 
Let us take, for instance, the plains of Lombardy, in Italy. 
I have sometimes been there when the heavens overhead 
were of the deepest blue, and not a cloud was to be seen. I 
have been there when the wind has blown towards Monte 
Rosa, in the Alps. You can see the effedt splendidly from 
the top of Milan Cathedral. I have seen the air which 
passed over the plains of Lombardy without a single trace 
of cloud ; and when it came in contadt with the mountain 
it was tilted up to a higher level and chilled, aqd the Alps 
