106 NOMOS. 



imponderable agencies, and connect light, heat, and 

 motion with certain material changes is not so 

 readily set aside. According to these views, the 

 solar light and heat could not travel to the earth 

 without a material track. According to these views, 

 even motion may be said to require a resisting 

 medium for its manifestation. Now this difficulty, 

 as we have said, is not so easily set aside ; for though 

 it is not yet proved that the light and heat and 

 motion of the heavenly bodies are analogous to the 

 light and heat and motion of which we have 

 spoken, yet are we obliged by the recognized rules 

 of philosophising to assume the analogy until we 

 know the contrary. In making this assumption, 

 moreover, we are proceeding from the known to the 

 unknown; for nothing whatever is known of the 

 causes of the light and heat and motion of the hea- 

 venly bodies. 



How, then, is this difficulty to be set aside, for 

 assuredly it is a great difficulty to suppose that space 

 is filled with a substance which is sufficiently real 

 and sufficiently ethereal for our purpose sufficiently 

 real to allow the performance of those definite mole- 

 cular changes which in one of their aspects we name 

 chemical, and sufficiently ethereal to oppose no re- 

 sistance to the free continuance of the motion of pro- 

 jection? Are we to suppose that the invisible con- 

 tents of space have a motion in the same direction, 

 and with the same velocity, as the heavenly bodies 

 themselves ; or are we to suppose that the heavenly 

 bodies are moved along, not merely by an initial 

 tangential impulse, but by that ever-acting force 



