A small flask is filled with coloured water, and stopped with a cork.  Through the cork passes a glass tube water-tight, the liquid standing at a certain height in the tube.  The flask and its tube resemble the bulb and stem of a thermometer.  Applying the heat of a spirit-lamp, the water rises in the tube, and finally trickles over the top.  Expansion by heat is thus illustrated.

Removing the lamp and piling a freezing mixture round the flask, the liquid column falls, thus showing the contraction of the water by the cold.  But let the freezing mixture continue to act:  the falling of the column continues to a certain point; it then ceases.  The top of the column remains stationary for some seconds, and afterwards begins to rise.  The contraction has ceased, and expansion by cold sets in.  Let the expansion continue till the liquid trickles a second time over the top of the tube.  The freezing mixture has here produced to all appearance the same effect as the flame.  In the case of water, contraction by cold ceases, and expansion by cold sets in at the definite temperature of 39 deg.  Fahr.  Crystallization has virtually here commenced, the molecules preparing themselves for the subsequent act of solidification, which occurs at 32 deg., and in which the expansion suddenly culminates.  In virtue of this expansion, ice, as you know, is lighter than water in the proportion of 8 to 9.[16]

A molecular problem of great interest is here involved, and I wish now to place before you, for the satisfaction of your minds, a possible solution of the problem:—­

Consider, then, the ideal case of a number of magnets deprived of weight, but retaining their polar forces.  If we had a mobile liquid of the specific gravity of steel, we might, by making the magnets float in it, realize this state of things, for in such a liquid the magnets would neither sink nor swim.  Now, the principle of gravitation enunciated by Newton is that every particle of matter, of every kind, attracts every other particle with a force varying inversely as the square of the distance.  In virtue of the attraction of gravity, then, the magnets, if perfectly free to move, would slowly approach each other.

But besides the unpolar force of gravity, which belongs to matter in general, the magnets are endowed with the polar force of magnetism.  For a time, however, the polar forces do not come sensibly into play.  In this condition the magnets resemble our water-molecules at the temperature say of 50 deg..  But the magnets come at length sufficiently near each other to enable their poles to interact.  From this point the action ceases to be solely a general attraction of the masses.  Attractions of special points of the masses and repulsions of other points now come into play; and it is easy to see that the rearrangement of the magnets consequent upon the introduction of these new forces may be such as to require a greater amount of room.  This, I take it, is the case