i88o.] 
On Water and Air . 
63 
sky-blue. If you drop a pin into one of the tanks, which 
contain 120,000 gallons each, you can see the pin lying at 
the bottom. I have thrown farthings into the tanks over 
and over again, and they are scarcely dimmed by the depth 
of 16 ft. of this beautiful chalk water ; and this water has 
many other things to recommend it. For instance, I have 
looked into the Report of the Registrar General for informa- 
tion with regard to the influence of temperature upon 
Thames water, and I have found that, in the early part of the 
month of June last year, the temperature of the Thames 
water rose from 6o° to 67° or 68°, and that the death-rate 
from a certain complaint was at first represented by the 
number 23, but it rose up in three or four weeks to 349, and 
this increase the Registrar General attributes entirely to the 
rise in the temperature of the Thames water. Now, one 
great advantage of this Canterbury water that comes from 
the body of the chalk is that its temperature is perfectly 
constant. It is always at an agreeable temperature of 51 0 
to 52° ; and if it is properly laid and conducted to the houses 
it cannot possibly be the propagator of any form of infectious 
disease. 
But now I pass on from the temperature of the Canter- 
bury water to show you the colour of the water. I have 
filled this long tube to half its depth with the water, so that 
half the beam of light which passes through the tube will 
go through the air, and the other half will go through the 
water. [The 15 feet tube, to which a reference had been 
made in an earlier part of the leCture, was used in this ex- 
periment. The beam of light, after passing through the 
tube, was received on a white screen. A light green tint 
was communicated by the water.] There we have a great 
deal of colour due to the purest water that we can find. 
Now I want to point out to you another physical property 
of water. The liquid state of matter such as water repre- 
sents is sometimes defined to be that state in which cohesion 
is absent, — a state in which the particles do not cohere, and in 
which they are perfectly mobile and are not held together 
by any force. I would ask you to remember that that is an 
erroneous definition of the liquid condition. The liquid 
condition is this ; — The little particles of water (and I shall 
tell you in a future leCture what these particles are called, 
but for the time being we shall call them “ particles ”) are 
gifted with the power of sliding over each other ; but, while 
gifted with that power, they can exercise a very great resist- 
ance to being torn asunder. We may measure the cohesion 
of water by a simple apparatus. Here is a plate of glass, 
