ets are
attached, and tapping the plate, you see the arrangement of the iron
filings in those magnetic curves which have been so long familiar to
scientific men (fig. 23).

[Illustration: Fig. 23.

N is the nozzle of the lamp; M a plane mirror, reflecting the beam
upwards. At P the magnets and iron filings are placed; L is a lens
which forms an image of the magnets and filings; and R is a totally
reflecting prism, which casts the image G upon the screen.]

(By a very ingenious device, Professor Mayer, of Hoboken, has
succeeded in fixing and photographing the magnetic curves. I am
indebted to his kindness for the annexed beautiful illustration, fig.
24.)

The aspect of these curves so fascinated Faraday that the greater
portion of his intellectual life was devoted to pondering over them.
He invested the space through which they run with a kind of
materiality; and the probability is that the progress of science, by
connecting the phenomena of magnetism with the luminiferous ether,
will prove these 'lines of force,' as Faraday loved to call them, to
represent a condition of this mysterious substratum of all radiant
action.

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,