i 88 o.] On Water and Air . 115 
sudden leap. When you attempt to boil it, it leaps almost 
out of the vessel in which it is contained, if it be freed from 
the air. Well, Sir William Grove found that he got, not 
this regular, tranquil, continuous boiling, but a series of 
sudden leaps in his boiling, due to a sudden generation of a 
quantity of steam ; and he found that he never could get a 
bubble of steam that condensed entirely in cold water or in 
the cold oil that he had above the water. There was always 
a little air ; and when he examined it he found it to be a 
particular portion of air,— that is, the nitrogen,— so that he 
never could get a bubble of steam without this bubble of 
gas. He came to the conclusion that if you could entirely 
free water of its air you never could get boiling at all. It 
would be heated up until it would be chemical)y decomposed 
instead of undergoing ordinary boiling. Well, so much for 
the tenacity of water and its non-compressibility. 
I have now to say one or two words more with regard to 
the colour of water hinted at in our last lefture. You know 
that on that occasion I formed one brilliant spedtrum upon 
the screen, and that I took this vessel— which consists of 
three cells— and filled the thin cell with a little liquid, and 
that that had hardlv any effeft upon the spetfxum. I then 
filled another cell, and that cut off a third of the spedlrum. 
I then filled the thickest cell, and it cut off practically the 
whole of the speCtrum. You had there an example of the 
effedt of the deepening of the layer of water in quenching 
the light. Now I am gcing to introduce to you a problem 
which is not usually ir oduced into lectures, but I think 
that it is one which you will easily understand, and 
which is worth underst ding. Here is a black liquid. It 
is, in fadt, a quantity c ink. The light shines upon it from 
the top of the house, an i it strikes the surface of the liquid, 
and as I look into it I see the image of the gaslights 
overhead : that is due to what is called surface reflection. 
The ink refleas light from its surface, but this is simply 
some of the light that falls upon it. The light that gives a 
body its colour must go into the body to a certain depth, 
and be ejeaed from the body minus some of the constituents 
of white light ; but the light which comes from the surface 
of the body is simply white light. Why is this ink black ? 
Simply because there is no ejeaion of light from the interior 
of the liquid. No light comes from within. You have only 
surface refleaion, and therefore the liquid is black. 
And now let us reason together. Suppose you stand upon 
a ship’s side and look down into water two or three thousand 
fathoms in depth— the deep, deep, perfealy pure sea. That 
