t upon. A cobweb spread above a blossom is sufficient to
protect it from nightly chill; and thus the aqueous vapour of our
air, attenuated as it is, checks the drain of terrestrial heat, and
saves the surface of our planet from the refrigeration which would
assuredly accrue, were no such substance interposed between it and the
voids of space. We considered the influence of vibrating period, and
molecular form, on absorption and radiation, and finally deduced, from
its action upon radiant heat, the exact amount of carbonic acid
expired by the human lungs.

Thus, in brief outline, were placed before you some ofthe results of
recent enquiries in the domain of Radiation, and my aim throughout has
been to raise in your minds distinct physical images of the various
processes involved in our researches. It is thought by some that
natural science has a deadening influence on the imagination, and a
doubt might fairly be raised as to the value of any study which would
necessarily have this effect. But the experience of the last hour
must, I think, have convinced you, that the study of natural science
goes hand in hand with the culture of the imagination. Throughout the
greater part of this discourse we have been sustained by this faculty.
We have been picturing atoms, and molecules, and vibrations, and
waves, which eye has never seen nor ear heard, and which can only be
discerned by the exercise of imagination. This, in fact, is the
faculty which enables us transcend the boundaries of sense, and
connect the phenomena of our visible world with those of an invisible
one. Without imagination we never could have risen to the conceptions
which have occupied us here today; and in proportion to your power of
exercising this faculty aright, and of associating definite mental
images with the terms employed, will be the pleasure and the profit
which you will derive from this lecture.

The outward facts of nature are insufficient to satisfy the mind. We
cannot be content with knowing that the light and heat of the sun
illuminate and warm the world. We are led irresistibly to enquire,
'What is light, and what is heat?' and this question leads us at once
out of the region of sense into that of imagination. [Footnote: This
line of thought was pursued further five years subsequently. See
'Scientific Use of the Imagination' in Vol. II.]

Thus pondering, and questioning, and striving to supplement that which
is felt and seen, but whi