i88o.] 
On Water and Air . 
253 
particle of iron filings here has two poles, and in virtue of 
this double polar action they so aft upon each other as to 
arrange themselves in this particular way. We have here 
an example of the aftion of what we call a polar force. 
We see that this polar force is capable of causing these 
small particles of iron to arrange themselves in definite 
curves. Now what I want you to understand is this : — 
Starting from this conception (the notion of a polar force 
given to those iron filings) the scientific man, who uses the 
eye of his mind far more than the eye of his body, sees in 
the power that builds those snow crystals together a kind of 
polar force. Were it not that in a room so warm as this 
it is difficult to operate upon ice, I could produce before you 
those beautiful crystals of water which, as I have said, are 
so lovely when you see them upon your window-panes on a 
cold morning. We have been operating upon these crystals, 
and have produced them in a variety of ways. But instead 
of water I will operate upon something different. Here we 
have various kinds of beautiful crystals. This is a crystal 
of our rock salt that we had in our last leCture ; and if you 
look at this you will see, as in a diamond, planes of shining 
surfaces. Those planes are the planes of cleavage, and we 
can cut the crystal parallel to those planes of cleavage and 
produce these beautiful shining surfaces. This rock salt is 
cleavable into beautiful cubes ; so with the diamond also. 
It is in the same way cleavable into masses corresponding 
to its own crystalline form. 
I will take a film of liquid, which I trust we shall be able 
to crystallise. Mr. Cottrell has here some very clean plates 
of glass, and he will pour upon one of the plates of glass a 
film of liquid that will be capable of crystallising. We may 
use water as the liquid in this experiment, but it will be 
more convenient to experiment with sal-ammoniac, or, as 
it is called, chloride of ammonium. Mr. Cottrell will pour 
a film of this liquid upon a glass plate, which he has now in 
his hand ; and he warms the plate for a little while in order 
to hasten the evaporation. Then he will place the plate in 
front of a microscope, and he will throw the image upon the 
screen. Here you see the particles suspended in the liquid, 
and I have no doubt that when the evaporation has been 
going on for a time you will find crystals starting through 
this mass. You will find an illustration of that wonderful 
architecture, by which the snow crystals are built up. There, 
you now can see the crystals forming. See how they dart 
across the field — those wonderful crystalline spears, which 
are illustrations of this building-power to which I have re- 
