EADIATIOX. 311 



and finally deduced from its action upon radiant heat the exact amount of car- 

 bonic acid expired by tlie human lunijfrf. 



Thus, in brief outline, have I placed before you scmie of the results of recent 

 inquiries in the domain of radiation, and my aim throughout has been to raise 

 in your minds distinct pliysieal imac^es of the various processes involved in our 

 researches. It is thon.',''lit V)y some tliat natural science hasadeadenini^ iulluenc^e 

 on the imaj^ination, and a doubt miglit fairly be raised as to tlie vahie of any 

 study which would necessarily have this effect. But the experience of the last 

 liour must, I think, have convinced you that the study of natural science goes 

 hand in hand with the culture of the imasfination. Throughout the greater part 

 of this discourse we have been sustained by this faculty ; we have been picttning 

 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 phenomena of our visible world with those of an invisible one. 

 Withfiut imagination we never could have risen to the conceptions which have 

 ocenpied ns liere to-day; and in proportion to yom* power, of exercising this 

 faculty aright, and of associating delinite mental images with t\w. terms employed, 

 will be the pleasure and the prcjfit which you will derive from this lecture. The 

 outward facts of nature are insufficient to satisfy the mind. We cannot be con- 

 tent 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 ns at once out of the region of sense into that of imagination. 



TluLs pondering, and asking, and striving to supplement that which is felt and 

 seen, but v.hich is incomplete, by something unfelt and unseen which is necessary 

 to its completeness, men of genius have in part discerned, not only the nature of 

 li»ht and heat, but also, through them, the general relationship of natural phe- 

 nomena. The power of natm-e is the power of motion, of which all its phenomena 

 are but special forms. It manifests itself in tangible and in intangible matter, 

 b.cing incessantly transferred from the one to the other, and incessantly trans- 

 formed l)y the change. It is as real in the waves of the ether as in the waves 

 of the sea, the latter being, in fact, nothing more than the heaped-uj) motion 

 of the former, for it is the calorific waves emitted by the sun which heat our 

 air, produce our winds, and hence agitate our ocean ; and whether they break in 

 foam upon the shore, or rub silently against the ocean's bed, or subside by the 

 mutual friction of their own parts, the sea-waves finally resolve themselves into 

 waves of ether, and thus regenerate the motion from which their tem])orary exist- 

 once was derived. This connection is typical. Nature is not an aggregate of 

 indepen<lent parts, but an organic whole. If you open a piano and sing into it 

 a certain string will res})ond. Change the pitch of your voice; the first string 

 ceases to vibrate, but another replies. Change again the pitch ; the first two 

 strings are silent, while another resounds. Now, in altering the pitch you sim- 

 ply change the form of the motion communicated by your vocal chords to tho 

 air, one string responding to one form and another to another. And thus is sen- 

 tient man sung unto by natm-e, while the optic, the auditory, and other nerves of 

 the human body are so many strings dill'erently tuned and responsive to diHerent 

 forms of the universal ])ower. 



