On Water and Air . 
52 
[January, 
St. Michael’s Cave at Gibraltar. They are so beautiful that 
it seems a kind of desecration to break them. Here we have 
beautiful stalactites produced by the evaporation of water 
containing a mineral in solution, and that mineral is what 
we call carbonate of lime. 
And now we have to examine something about this car- 
bonate of lime. It is a body compounded of carbonic acid 
and lime. Every boy knows what lime is. I have here 
some quick-lime in this vessel. In order to show the 
influence of boiling, I have here water boiling hi two different 
flasks. One of them has, perhaps, been boiling an insuffi- 
cient length of time to deposit all its solid matter, but I 
think you will see that there is a very considerable differ- 
ence between these two flasks. Now, these are two different 
kinds of water. One of the flasks contains water for which 
I am indebted to a distinguished engineer, Mr. Homersham, 
who has made certain water- works at Canterbury. It is 
the Canterbury water, which has been softened by a pro- 
cess that we shall learn about by-and-bye. You see 
that the water in this flask is perfectly clear, because 
all the mineral matter has been removed from it before 
boiling; but the water in the other flask, which is the 
tap-water of our house, is thick and turbid. . You see 
that the mineral matter has been let loose, and is forming 
a kind of mud, in point of faCt. If I place the flask in 
this beam of light those at the right and the left will see 
that sparkling stuff, which is the mineral matter — the car- 
bonate of lime — which has been liberated by the boiling ; 
and it is this stuff which, when deposited upon the interior 
surface of our kettle, produces that incrustation to which I 
have referred. This open vessel merely shows the effeCt of 
evaporation. A quantity of water from St. Govor’s Well in 
Kensington Gardens was placed in this basin this morning 
and evaporated, and there is the substance which gives the 
water its peculiar taste and its peculiar medical value. 
This is the substance which, when it is liberated by evapor- 
ation, produces that red splash which you see when you 
look at the well. . 
Now, we want fully to understand the meaning of this 
phrase that I have used— carbonate of lime. Carbonate of 
lime is, as I have said, a mixture of two distinct substances 
—carbonic acid gas, and lime. I will just remind you of 
what this carbonic acid is. There is a quantity of it here. 
My friend, Professor Dewar, is kind enough to help me, and 
here he has been exhausting a glass globe, and here is another 
exhausted globe. At the present time the two globes 
balance each other. Now, if I allow carbonic acid to enter 
