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.


§ 2. _Theory of Crystallization._