54 
On Water and Air . 
[January, 
mix this lime with water, the water dissolves a certain 
amount of lime, and then we have lime-water ; and here, 
thanks to Mr. Homersham, I have a specimen of lime-water 
which is obtained from the Canterbury works in a way 
which I will presently describe to you. Lime-water is 
produced, as I have said, by a solution of ordinary lime in 
water. Lime is not very soluble. It requires, about 70 
gallons of water to dissolve a single pound of lime. But 
here we have our lime-water; and if you were to taste this 
solution you would find it very pungent to the taste, because 
of the lime which is dissolved in it. Now, I wish to make 
carbonate of lime in your presence, and I will take this 
beaker, as it is called, and I will pour a portion of this lime- 
water into the beaker. There it is. Mr. Cottrell has here 
an apparatus for making this substance which we call car- 
bonic acid — that heavy gas which was poured out a moment 
ago in your presence. I intend to let the carbonic acid 
bubble through the lime water. You see, I . am going to 
the A B C of the question, and I want to bring you from 
the A B C as far up as we can climb. But at first I am 
going to the elements of the question, and those who 
honour these lectures with their presence will, of course, 
remember that the lectures are addressed to boys and girls, 
and that learning or depth would be very much out of place 
here. 
Now, I have the carbonic acid in this vessel, and I will 
bring it into contact with the lime-water. You will then 
find that that clear liquid will become milky. Mr. Cottrell 
will allow the gas to bubble through the water, and you 
will see in a very short time that the liquid is rendered 
milky. I have placed this black cloth here [behind the 
vessel containing the lime-water] . It is one of the devices 
that we employ in order to make things visible. The change 
in the appearance of the liquid is due to the formation of 
this carbonate of lime, which is an insoluble white powder, 
and which becomes diffused through the water, producing 
that milkiness which you see before you. 
Instead of making the carbonic acid in the way in which 
it is made there, it may be made by means of marble or 
chalk, which are themselves carbonates of lime. Carbonate 
of lime is a compound of lime and carbonic acid ; and if 
you pour upon the marble or chalk an acid which is stronger 
than the carbonic acid, the carbonic acid will be liberated. 
Every boy may do this for himself. Here I put a few bits 
of chalk into this vessel, and if I pour upon them a little 
acid, you will find that it effervesces. I do this for the 
