RADIATION. 71 



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 

 of the results of recent enquiries in the domain of 

 Eadiation, and my aim throughout has been to raise in 

 your minds distinct physical images of the various pro- 

 cesses 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 N the ima- 

 gination. 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 ima- 

 gination. This, in fact, is the faculty which enables us 

 to transcend the boundaries of sense, and connect the 

 phenomena of our visible world with those of an in- 

 visible 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. 

 6 



