n. Their identity is only an idea, for we put off
our bodies many times during our lives, and dress in new suits of bones
and muscles.

"Thou art not thyself;
For thou exist'st on many a thousand grains
That issue out of dust."

If it is true that we understand ourselves but imperfectly in health,
this truth is more signally manifested in disease, where natural actions
imperfectly understood, disturbed in an obscure way by half-seen causes,
are creeping and winding along in the dark toward their destined issue,
sometimes using our remedies as safe stepping-stones, occasionally, it
may be, stumbling over them as obstacles.

I propose in this lecture to show you some points of contact between our
ignorance and our knowledge in several of the branches upon the study
of which you are entering. I may teach you a very little directly, but I
hope much more from the trains of thought I shall suggest. Do not expect
too much ground to be covered in this rapid survey. Our task is only
that of sending out a few pickets under the starry flag of science to
the edge of that dark domain where the ensigns of the obstinate rebel,
Ignorance, are flying undisputed. We are not making a reconnoissance
in force, still less advancing with the main column. But here are a few
roads along which we have to march together, and we wish to see clearly
how far our lines extend, and where the enemy's outposts begin.

Before touching the branches of knowledge that deal with organization
and vital functions, let us glance at that science which meets you at
the threshold of your study, and prepares you in some measure to deal
with the more complex problems of the living laboratory.

CHEMISTRY. includes the art of separating and combining the elements of
matter, and the study of the changes produced by these operations. We
can hardly say too much of what it has contributed to our knowledge of
the universe and our power of dealing with its materials. It has given
us a catalogue raisonne of the substances found upon our planet, and
shown how everything living and dead is put together from them. It is
accomplishing wonders before us every day, such as Arabian story-tellers
used to string together in their fables. It spreads the sensitive film
on the artificial retina which looks upon us through the optician's lens
for a few seconds, and fixes an image that will outlive its original.
It questions the light of the sun, and detects the vaporiz