120 
On Water and Air. 
[February, 
less gas flame was held underneath the beam at the focus b, 
immediately destroying its luminosity and producing a space 
of intense blackness.] Now you see these extraordinary 
black wreaths that go up into that beam. What are they ? 
You would imagine that they were the smoke of the flame, 
would you not ? But they are not at all. If I were to throw 
smoke into the beam, the smoke would appear quite white. 
I will try. We will strike a match and allow it to burn 
beneath the beam. [The smoke of a burning match was 
allowed to ascend into the eledtric beam, and appeared as a 
white cloud.] The blackness which you saw was not 
smoke at all. The black appearance was due to the air 
having been deprived of its floating matter. That floating 
matter is combustible, and when it is burnt away, and the 
air from which it has been burnt ascends through the beam, 
this blackness is produced as if there were a dense black 
smoke, although, as I have said, it is not smoke at all, for 
smoke actually produced whiteness instead of blackness. 
Now, I want to enable you to compare the water of 
Canterbury with the water from our own cistern. I 
explained to you the nature of the Canterbury water in our 
last ledture. We have some of it in this flask, and in this 
other flask we have some of the water from our tap which 
is supplied, I believe, by the Grand Junction Water Com- 
pany. For a long time past we have had but very little 
rain until to-day, and the consequence is that the tap water 
at the present time is exceedingly good. It is comparatively 
clear, for we have it now at its best. If you read the 
reports in “ The Times ” and in other newspapers, you will 
find that the water supplied to London is now very good 
indeed, and very free from suspended particles. A beam of 
light will be made to pass through these flasks, so that you 
may judge of the comparative purity of the water. I think 
you will find a great predominance of purity on the part of 
the Canterbury water. I am now going to light a very 
famous candle — the Jablochkoff candle as it is called. 
Right and left of the candle are placed two lenses so as to 
concentrate the light within the flasks. [The two flasks 
were illuminated as described, the course of the rays of 
light being perceptible in both flasks.] If the water were 
absolutely pure you would see no track of the beam in it. 
The green which you see in the flasks is due to the suspended 
matter of the water. In the Canterbury water you have a 
considerable amount of suspended matter, but there is less 
than in the tap water, notwithstanding the fadt that the tap 
water is now in its very best condition. 
