RADIATION. 53 
then briefly dwelt upon. A cobweb spread above a blos- 
som is sufficient to protect it from nightly chill; and thus 
the aqueous vapor 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 of 
the results of recent inquiries in the domain of radiation, 
and my aim throughout hys 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 experi- 
ence 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 en- 
ables us to transcend the boundaries of sense, and connect 
the phenomena of our visible world with those of an invis- 
ible one. Without imagination we never could have risen 
to the conceptions which have occupied us here to-day; and 
in proportion to vour 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 
inquire, ' What is light, and what is heat?" and this ques- 
tion leads us at once out of the region of sense into that of 
imagination.* 
This line of thought was pursued further five years subse-, 
quently, gee " Scientific Use of fhe Imagination ". 
