260 On Water and Air. [April, 
The power that melts them is those invisible rays that we 
referred to in our last lecture. 
Now, I also want to show you a peculiar aCtion on the 
part of water, which is of the utmost consequence in nature. 
Almost all bodies are expanded upon being heated. Mr. 
Cottrell has prepared a flask of water, and he intended me 
to heat it with a spirit lamp, so that you would see the 
expansion of the water in the tube which goes through the 
cork of the flask. The water would have expanded, and 
would have risen in the tube, and you would have seen on 
the screen the image of the water trickling over the top of 
the tube. I would have then removed the spirit-lamp which 
caused the water to expand, and would have surrounded the 
flask with a freezing mixture. Very soon you would see the 
water contracting in consequence of the cold. It would 
gradually contract and go down to a certain point, and then 
it would cease to contract. Then, if I continued the appli- 
cation of the cold, the water would begin again to expand. 
Up to a certain point the water would be contracted by cold. 
Beyond that point, the cold would aCt exactly as heat would 
aCt, and cause expansion ; and I intended to lift this column 
of water by expanding it by means of cold, so that you would 
see it trickling over the top of the column through the aCtion 
of cold instead of through the aCtion of heat, as previously. 
Water contracts until the temperature gets down to about 
4 0 C., or 39° F. That is about y° above the point where it 
freezes. That point corresponds to what they call the 
maximum density of water. Then if you continue the cold, 
it expands. What does this expansion mean ? It is simply 
a preparation for the aCt of crystallisation which converts 
water into ice. At this particular point, 39 0 F., the mole- 
cules, although still in the liquid condition, are beginning to 
aCt upon each other, so as to try to build themselves, as it 
were, into the crystalline form. The crystal of water re- 
quires more room than the water itself. When you lower 
the temperature so as to bring it down to 3 2 0 F., at which 
point water freezes, then the expansion is sudden and so 
powerful as to rend everything opposed to it. Here is a 
thick bombshell of cast-iron, for which I am indebted to my 
friend, Professor Abel. Similar bombshells were filled with 
water and screwed tightly down, and placed in a freezing 
mixture in front of the table before the leCture commenced ; 
and those explosions which you have heard during the 
leCture are, I anticipate, due to the bursting of those bomb- 
shells in consequence of the force exerted by the water in 
passing from the liquid condition into the solid condition. 
