246 
On Water and An. 
[April, 
ON WATER AND AIR.* 
By John Tyndall, D.C.L., LL.D., F.R.S., 
Professor of Natural Philosophy at the Royal Institution 
of Great Britain. 
Lecture IV. 
% N our last Leaure I tried to explain to you that the rays 
jjf of the sun that are really influential in producing eva- 
coration are rays which are entirely incompetent to 
excite P vision. They are what are called invisible rays, and, 
lest I should have been misunderstood, I think I will dwe 
for a moment upon this point of visible and invisible rays 
I wflUake here, as I have already done, a slice of white 
St and you shall see it, in the first instance, upon the 
screen There it is ; and now we turn this beam aside and 
it through a body which we call a prism. That will 
separate the different colours of that white light the one 
t the other and you will have upon the screen that 
beautiful exhibition St we call the spedtrum. There it xs 
Now I will add another prism to this single one, and in that 
T hnne to set, as we have formerly done, a large 
st^dtrum.^ [A second prism was introduced.] Here, then, 
is this beautiful display of colour produced by a slice of 
rfrdinarv white light. Now, I want you to understand that 
all these lovely colours emanate from this thing that I have 
called our little “ domestic sun,” the so-called eledtric hg . 
Now suppose that I were not gifted with the sense of vision, 
l °. nonose me to be gifted with an extreme sensitiveness 
as regards heat ; then I might walk through that spedtrum, 
A r g poort to vou what I should experience supposing my- 
“it o’be . blS man. I -11 simply walk through beg.h- 
nine at this dark portion of the spedtrum [adjacent to the 
nl __ nr at least, beyond the spedtrum altogether. Sup- 
posing my sense of warmth to be exceedingly delicate, I 
Konld sav to you that I am now receiving a very poweiful 
amounted heat from that eledtric lamp. You do not see 
. „ . „ . Course of Six Leftures adapted to a Juvenile Auditory, de, ! pered 
.. the Royaf Institution of Great Britain, Christmas, 1879. Specially re- 
ported for “The Journal of Science.” 
