6o 
On Water and Air. 
[January, 
shall not be able to produce any lather in the hard water of 
the Royal Institution by the amount of soap that I com- 
municate to it. You will find, perhaps, a little lather — a few 
bubbles at first, but they will speedily disappear. You will 
find that upon the soft water a beautiful lather of soap 
bubbles is formed, and on the other there will be a very 
greasy mass, which will be due to the combination of a 
certain acid in the soap with the lime of the water. The 
acid is called margaric acid, and the curdy mass which is 
formed is called margarate of lime. Here, from a very 
small amount of soap, you see a very beautiful lather is pro- 
duced in a moment in the soft water. A permanent lather 
covers the bowl of water. I go to the other basin and 
operate in the same fashion, and I obtain some few soap 
bubbles at the top, but they will soon disappear ; and I see 
floating on the top and mixing with the water a quantity of 
this curdy, unpleasant-looking stuff that I have spoken of as 
margarate of lime. Now, with a glass tube, I will blow 
into this lather [that produced from the soft water] , and I 
get a very splendid and permanent heap of bubbles. In the 
same manner I blow into the basin containing the tap- 
water, and you see how few bubbles are produced, and their 
rapid disappearance. 
I have now to say a word upon some of the other proper- 
ties of water, and first we will take its colour. The colour 
of water is a delicate blue. If you pour water into an ordi- 
nary glass you see nothing of the colour of the water, but in 
large masses you can see the colour, or, at least, the colour 
can be revealed. For the purpose of showing the colour of 
water I have here a tube 15 feet long, and that tube is 
partly filled with water. But before we deal any farther 
with the colour of water, I should just like to say three or 
four words with regard to colour in general, and for that 
purpose I will show you, if I can, on what all colour depends. 
I have here in this camera a little artificial sun [referring 
to the eledtric lamp] . If we take the light of the sun at 
noonday on a cloudless day, we call the light which it gives 
white light. Sometimes, as the sun sinks towards the 
horizon, his rays become red ; but at noonday, when there 
are no clouds, the light is white. Our little domestic sun 
(the eledtric lamp) is precisely of that kind. We cause the 
light from our little domestic sun to impinge upon our screen. 
I take for this purpose simply a little strip of light — a slice 
of light, if I may use the expression — and there that slice of 
light is thrown upon the screen and produces that white 
redtangle which you see. I will interpose in the path of the 
