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 ha-s 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 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 

 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. See " Scientific Use of the Imagination". 



