i88 On Water and Air. [March, 
We will now pass on to the consideration of other things. 
I trust that you have a clear conception of what is meant 
by evaporation and by vapour. I want to proceed to the 
investigation of the particular agent which produces the 
aqueous vapour of our air, and which produces all our rains, 
snow, and hail. Of course I need not tell you that the 
agent that raises the vapour from the surface of the sea is 
the sun. The sun pours down his heat upon the sea and 
warms the water, and this water rises into the air as vapour, 
and when it is condensed in the air it falls as rain. 
The investigation which I propose to place before you at 
the present moment is this : — How does the sun bring this 
about ? Are the whole of the sun’s rays influential in pro- 
ducing this evaporation ? Are those rays which enable us 
to see bodies influential in producing it ? . Are those rays 
that produce vision influential in evaporating the water of 
the ocean ? All these questions we have now to examine ; 
and I trust that you will give me your patience if I try to 
examine them to the very bottom. 
In the first instance I will try to produce before you what 
I will call an artificial sun. [A powerful eleCtric current 
from a magneto-machine was set in aCtion, producing an 
intense light and heat.] This, then, is a little artificial sun 
with which we are going to operate, and that it may not 
annoy you too much we have enclosed the lamp which regu- 
lates this current in this kind of camera. This light is 
produced by a gas-engine downstairs, which works a beau- 
tiful machine which has been kindly presented to the Insti- 
tution by Mr. William Siemens. The rays from this little 
sun shall impinge upon a reflector such as I hold in my 
hand. They will be gathered up by the refleCtor and thrown 
forward. Here is our camera (Fig. 21 ), and I will place 
within it this reflector M, and through the aperture in front 
of the camera we will reflect the rays emanating from the 
intensely-heated carbon-poles p and p', and there I obtain an 
intense focus of heat and light at F. Now I want to show 
you that that beam is capable of producing ignition. For 
instance, I hold this piece of paper at the focus F. . [The 
paper immediately took fire.] Look at that ! That is done 
by the pure radiation — not by any flame. The paper is 
ignited by the condensed radiation from our little domestic 
sun. Here is a metal, and I dare say that this metal will 
burn just like the sheet of paper, if I place it in the focus of 
the light. [Apiece of thin sheet zinc, upon being held at 
the focus, immediately burnt with its characteristic greenish 
blue flame.] We have the zinc burning like so much com- 
