MAGNETIC ROTATION OF LIGHT: — GENERAL CONSIDERATIONS. 
19 
The natural condition of the magnet and quartz, and the constrained condition of 
the helix and heavy glass, form the link of the analogy in one direction ; whilst the 
supposition of currents existing in the magnet and helix, and only a tendency or 
tension to currents existing in the quartz and heavy glass, supplies the link in the 
transverse direction. 
2237. As to those bodies which seem as yet to give no indication of the power over 
light, and therefore none of the assumption of the new magnetic conditions ; these may 
be divided into two classes, the one including air, gases and vapours, and the other 
rock crystal, Iceland spar, and certain other crystalline bodies. As regards the latter 
class, I shall give, in the next series of these researches, proofs drawn from phenomena 
of an entirely different kind, that they do acquire the new magnetic condition ; and 
these being so disposed of for the moment, I am inclined to believe that even air and 
gases have the power to assume the peculiar state, and even to affect light, but in a 
degree so small that as yet it has not been made sensible. Still the gaseous state is 
such a remarkable condition of matter, that we ought not too hastily to assume that 
the substances which, in the solid and liquid state, possess properties even general in 
character, always carry these into their gaseous condition. 
2238. Rock-salt, fluor-spar, and, I think, alum, affect the ray of light; the other 
crystals experimented with did not ; these are equiaxed and singly refracting, the 
others are unequiaxed and doubly refracting. Perhaps these instances, with that of 
the rotation of quartz, may even now indicate a relation between magnetism, electri- 
city, and the crystallizing forces of matter. 
2239. All bodies are affected by helices as by magnets, and according to laws 
which show that the causes of the action are identical as well as the effects. This 
result supplies another fine proof in favour of the identity of helices and magnets, 
according to the views of Ampere. 
2240. The theory of static induction which I formerly ventured to set forth (1161, 
&c.), and which depends upon the action of the contiguous particles of the dielectric 
intervening between the inductric and the inducteous bodies, led me to expect that 
the same kind of dependence upon the intervening particles would be found to exist 
in magnetic action ; and I published certain experiments and considerations on this 
point seven years ago (1709 — 1736.). I could not then discover any peculiar con- 
dition of the intervening substance or diamagnetic ; but now that I have been able to 
make out such a state, which is not only a state of tension (2227-), but dependent 
entirely upon the magnetic lines which pass through the substance, I am more than 
ever encouraged to believe that the view then advanced is correct. 
2241. Although the magnetic and electric forces appear to exert no power on the 
ordinary or on the depolarized ray of light, we can hardly doubt but that they have 
some special influence, which probably will soon be made apparent by experiment. 
Neither can it be supposed otherwise than that the same kind of action should take 
place on the other forms of radiant agents as heat and chemical force. 
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