He knew that polarized light was a most subtle and delicate
investigator of molecular condition. He used it in 1834 in
exploring his electrolytes, and he tried it in 1838 upon his
dielectrics. At that time he coated two opposite faces of a glass
cube with tinfoil, connected one coating with his powerful electric
machine and the other with the earth, and examined by polarized
light the condition of the glass when thus subjected to strong
electric influence. He failed to obtain any effect; still he was
persuaded an action existed, and required only suitable means to
call it forth.

After his return from Switzerland he was beset by these thoughts;
they were more inspired than logical: but he resorted to magnets and
proved his inspiration true. His dislike of 'doubtful knowledge'
and his efforts to liberate his mind from the thraldom of hypotheses
have been already referred to. Still this rebel against theory was
incessantly theorising himself. His principal researches are all
connected by an undercurrent of speculation. Theoretic ideas were
the very sap of his intellect--the source from which all his
strength as an experimenter was derived. While once sauntering with
him through the Crystal Palace, at Sydenham, I asked him what
directed his attention to the magnetization of light. It was his
theoretic notions. He had certain views regarding the unity and
convertibility of natural forces; certain ideas regarding the
vibrations of light and their relations to the lines of magnetic
force; these views and ideas drove him to investigation. And so it
must always be: the great experimentalist must ever be the habitual
theorist, whether or not he gives to his theories formal
enunciation.