SIX LECTURES ON LIGHT. 



23 



weight, but retaining thtir polar forces. If 

 we had a 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 

 is .that every particle of matter attracts every 

 other particle with a force varying as the in- 

 inverse square of the distance. In virtue of 

 the attraction of gravity, then, the magnets, 

 if perfectly free to move, would slowly ap- 

 proach each other. 



But besides the unjsolar force of gravity, 

 which belongs to matter in general, the mag- 

 nets are endowed with the polar force of 

 magnetism. For a time, however, the polar 

 forces do not sensibly come into play. In 

 this condition the magnets resemble our water 

 molecules at the temperature say of 50". 



E 



; with the force of contraction until the freezing 

 j temperature is attained. Here the polar 

 | forces suddenly and finally gain the victory. 

 { The molecules close up and form solid crys- 

 I tals, a considerable augmentation of volume 

 | being the immediate consequence. 



\Ve can still further satisfy the intellect by 

 I showing that these conceptions can be real- 

 ized by a model. The molecule of water is 

 composed of two atoms of hydrogen, united 

 to one of oxygen. We may assume the mole- 

 cule built up of these atoms to be pyramidal. 

 Suppose the triangles in Fig. 8 to be drawn 

 touching the sides of the molecule, and the 

 disposition of the polar forces to be that indi- 

 cated by the letters ; the points marked A 

 being attractive, and those marked R repel- 

 lent. In virtue of the general attraction of 

 the molecules, let them be drawn towards the 



But the magnets come at length sufiici"Tily 

 near each other to enable their poles to tiler- 

 act. From this point the action cease* to be 

 a general attraction of the masses. An at- 

 traction of special points of the masses and a 

 repulsion of other points now come into play; 

 and it is easy to see that the rearrangement 

 of the magnets consequent upon the intro- 

 duction of these new forces may be suth as 

 to require a greater amount of room. This, 

 I take it, is the case with our water-mole- 

 cules. Like the magnets, they approach each 

 other as wholes ^ until the temperature ^9 is 

 reached. Previous to this temperature, 

 doubtless, the polar forces had begun to act, 

 and at this temperature their action exactly 

 balances the contraction due to cold. At 

 lower temperatures the polar forces predomi- 

 nate. But they carry on a gradual struggle 



positions marked by the full lines, and then 

 suppose the polar attractions and repulsions 

 to act. A will turn towards A', and R will 

 retreat from R x . The molecules will be caused 

 lo lotate, their final positions being that shown 

 by the dotted lines. But the circle surround 

 ing the latter is larger than that surrounding 

 the full lines, which shows that the molecules 

 in their new positions require more room. In 

 this v.ay we obtain an image of the molecular 

 mechai ism active in the case of water. The 

 demand for more room is made with an energy 

 sufficient to overcome all ordinary resistances. 

 Your lead pipes yield readily to this power; but 

 iron does the same, and bomb-shells, as you 

 know, can be burst by the freezing of water. 

 Thick iron bottles filled with water and placed 

 in a freezing mixture are shivered into frag- 

 ments by the resistless vigor of molecular force. 



