RADIATION 77 



general influence on tlie temperature of tlie earth, were 

 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 ac- 

 crue 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 Eadiation, 

 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 sci- 

 ence 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 fac- 

 ulty. 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 

 to transcend the boundaries of sense, and connect the phe- 

 nomena 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 to-day; and 



