hich the molecular architecture of crystals was an
incessant subject of mental contemplation, gave a tinge and bias to my
subsequent scientific thought, and their influence is easily traced in
my subsequent enquiries. For example, during nine years of labour on
the subject of radiation, heat and light were handled throughout by
me, not as ends, but as instruments by the aid of which the mind might
perchance lay hold upon the ultimate particles of matter.

Scientific progress depends mainly upon two factors which incessantly
interact--the strengthening of the mind by exercise, and the
illumination of phenomena by knowledge. There seems no limit to the
insight regarding physical processes which this interaction carries in
its train. Through such insight we are enabled to enter and explore
that subsensible world into which all natural phenomena strike their
roots, and from which they derive nutrition. By it we are enabled to
place before the mind's eye atoms and atomic motions which lie far
beyond the range of the senses, and to apply to them reasoning as
stringent as that applied by the mechanician to the motions and
collisions of sensible masses. But once committed to such
conceptions, there is a risk of being irresistibly led beyond the
bounds of inorganic nature. Even in those early stages of scientific
growth, I found myself more and more compelled to regard not only
crystals, but organic structures, the body of man inclusive, as cases
of molecular architecture, infinitely more complex, it is true, than
those of inorganic nature, but reducible, in the long run, to the same
mechanical laws. In ancient journals I find recorded ponderings and
speculations relating to these subjects, and attempts made, by
reference to magnetic and crystalline phenomena, to present some
satisfactory image to the mind of the way in which plants and animals
are built up. Perhaps I may be excused for noting a sample of these
early speculations, already possibly known to a few of my readers, but
which here finds a more suitable place than that which it formerly
occupied.

*****

Sitting, in the summer of 1855, with my friend Dr. Rebus under the
shadow of a massive elm on the bank of a river in Normandy, the
current of our thoughts and conversation was substantially this: We
regarded the tree above us. In opposition to gravity its molecules
had ascended, diverged into branches, and budded into innumerable
leaves. What caused them