On Water and Air . 
195 
1880.] 
then interposed. The opaque liquid consisted of a solution 
of iodine in bisulphide of carbon.] You saw a moment ago 
that the interposition of the alum cell completely stopped 
the boiling ; but now, when we put in this cell of opaque 
liquid, the boiling continues as before. So here we are boil- 
ing the water by the invisible rays and not by the visible 
rays, and this is the point that I am labouring to bring you 
to. I say it advisedly, for it has been some labour. I have 
been labouring to bring you up to this point -that it is not 
the luminous rays of the sun that cause the ocean to evapo- 
rate and that cause the vapour to go into the atmosphere, 
but that it is the invisible heat-rays of the sun that produce 
this effedt. If I wished to show you various experiments in 
connection with these non-luminous rays, I could show you 
that they have power to produce all the effects of ignition 
and combustion which we have already produced at the 
focus of the full beam. Just allow me to make one such 
experiment. Mr. Cottrell will give me a beam. Here is a 
focus produced in the air by the reflection of the rays. If 
now I take a piece of black paper, and hold it in the focus of 
the rays, you see that it instantly bursts into a blaze. We 
will now take the opaque solution which will destroy all the 
light, and place it in the beam. I now interpose a piece of 
black platinum, and hold it in the perfectly dark focus of the 
rays, where there is no luminosity whatever, and you find 
that the platinum is raised to vivid incandescence. So if I 
place a piece of blackened zinc in the same position, it 
immediately bursts into flame. Again we place our black- 
ened paper in this invisible focus, and it is immediately set 
on fire. You see the paper bursts into flame at this focus, 
which is entirely without light. 
And here we come to the conclusion, with regard to the 
origin of our rains, and hails, and snows, that all these are 
due to the sun, and that they are mainly and . almost 
exclusively due to that portion of the sun’s rays which does 
not at all administer to vision — the invisible rays of the sun 
of which you would not have known except by means of the 
experiments which we have made, and which were first made 
by the celebrated Sir William Herschel. 
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