It is not, however, the magnetic curves, as such, but their relationship to theoretic conceptions, that we have now to consider.  By the action of the bar magnet upon the needle we obtain the notion of a polar force; by the breaking of the strip of magnetized steel we attain the notion that polarity can attach itself to the ultimate particles of matter.  The experiment with the iron filings introduces a new idea into the mind; the idea, namely, of structural arrangement.  Every pair of filings possesses four poles, two of which are attractive and two repulsive.  The attractive poles approach, the repulsive poles retreat; the consequence being a certain definite arrangement of the particles with reference to each other.

Sec. 2. Theory of Crystallization.

Now this idea of structure, as produced by polar force, opens a way for the intellect into an entirely new region, and the reason you are asked to accompany me into this region is, that our next inquiry relates to the action of crystals upon light.  Prior to speaking of this action, I wish you to realise intellectually the process of crystalline architecture.  Look then into a granite quarry, and spend a few minutes in examining the rock.  It is not of perfectly uniform texture.  It is rather an agglomeration of pieces, which, on examination, present curiously defined forms.  You have there what mineralogists call quartz, you have felspar, you have mica.  In a mineralogical cabinet, where these substances are preserved separately, you will obtain some notion of their forms.  You will see there, also, specimens of beryl, topaz, emerald, tourmaline, heavy spar, fluor-spar, Iceland spar—­possibly a full-formed diamond, as it quitted the hand of Nature, not yet having got into the hands of the lapidary.

[Illustration:  Fig. 24.]

These crystals, you will observe, are put together according to law; they are not chance productions; and, if you care to examine them more minutely, you will find their architecture capable of being to some extent revealed.  They often split in certain directions before a knife-edge, exposing smooth and shining surfaces, which are called planes of cleavage; and by following these planes you sometimes reach an internal form, disguised beneath the external form of the crystal.  Ponder these beautiful edifices of a hidden builder.  You cannot help asking yourself how they were built; and familiar as you now are with the notion of a polar force, and the ability of that force to produce structural arrangement, your inevitable answer will be, that those crystals are built by the play of polar forces with which their molecules are endowed.  In virtue of these forces, molecule lays itself to molecule in a perfectly definite way, the final visible form of the crystal depending upon this play of its ultimate particles.