Introduction xv

he permitted himself to use to me." The required apology
came, frank and full, creditable, I thought, alike to the
prime minister and the philosopher.

Considering the enormous strain imposed on Faraday's
intellect, the boy-like buoyancy even of his later years was
astonishing. He was often prostrate, but he had immense
resiliency, which he brought into action by getting away from
London whenever his health failed. I have.already indicated
the thoughts which filled his mind during the evening of his
life. He brooded on magnetic media and lines of force; and
the great object of the last investigation he ever undertook
was the decision of the question whether magnetic force
requires time for its propagation. How he proposed to attack
this subject we may never know. But he has left some
beautiful apparatus behind; delicate wheels and pinions, and
associated mirrors, which were to have been employed in the
investigation. The mere conception of such an inquiry is an
illustration of his strength and hopefulness, and it is impossible
to say to what results it might have led him. But the work
was too heavy for his tired brain. It was long before he could
bring himself to relinquish it, and during this struggle he often
suffered from fatigue of mind. It was at this period, and
before he resigned himself to the repose which marked the
last two years of his life, that he wrote to me the following
letter—one of many priceless letters now before me—which
reveals, more than anything another pen could express, the
state of his mind at the time. I was sometimes censured in
his presence for my doings in the Alps, but his constant reply
was, " Let him alone, he knows how to take care of himself."
In this letter, anxiety on this score reveals itself, for the $rst
time.

" Hampton Court, August i, 1864.

" MY DEAR TYNDALL,—I do not know whether my letter
will catch you, but I will risk it, though feeling very unfit
to communicate with a man whose life is as vivid and active
as yours; but the receipt of your kind letter makes me to
know that though I forget, I am not forgotten, and though
I am not able to remember at the end of a line what was said
at the beginning of it, the imperfect marks will convey to you
some sense of what I long to say. We had heard of your
illness through Miss Moore, and I was therefore very glad
to learn that you are now quite well; do not run too many
risks or make your happiness depend too much upon dangers,