448 THE PRINCIPLES OF SCIENCE. 



fication by observation, and almost free from any re- 

 strictions afforded by present knowledge. We might attri- 

 bute various shapes to the whole body of adamantine 

 medium, and the consequences would be various. But 

 there is this value in such speculations, that they draw 

 attention to the finiteness of our knowledge. We cannot 

 deny the possible truth of such an hypothesis, nor can we 

 place a limit to the scientific imagination in the framing 

 of other like hypotheses. It is impossible, indeed, to 

 follow out our scientific inferences without falling into 

 speculation. If heat be radiated into outward space it 

 must either proceed ad infinitum, or it must be stopped 

 somewhere. In the latter case we fall upon Rankine's 

 hypothesis. But if the material universe consist of a finite 

 collection of heated matter situated in a finite portion of 

 an infinite adamantine medium, then either this universe 

 must have existed for a finite time, or else it must have 

 cooled down during the infinity of past time indefinitely 

 near to the absolute zero of temperature. I objected to 

 Lucretius' argument against the destructibility of matter, 

 that we have no knowledge whatever of the laws accord- 

 ing to which it would undergo destruction. But we do 

 know the laws according to which the dissipation of heat 

 appears to proceed, and the conclusion inevitably is that a 

 finite heated material body placed in a perfectly cold 

 infinitely extended medium would in an infinite time 

 become infinitely approximated to zero. Now our own 

 world is not yet cooled down near to zero, so that physical 

 science seems to place us in the dilemma of admitting 

 either the finiteness of past duration of the world, or else 

 the finiteness of the portion of medium in which we exist. 

 In either case we become involved in metaphysical and 

 mechanical difficulties surpassing our mental powers. 



