Modes of Physical Force. 1 05 



which one of the physical forces has in turn been taken 

 as the starting point of enquiry. 



Much, however, remains to be done before the 

 subject is exhausted. Much, very much more a good 

 deal than is set forth in the broad resume of the argu- 

 ment which has been given has been done by the 

 author of the Correlation of the Physical Forces, but 

 not all. It is left undecided, in particular, whether the 

 view propounded is of universal or only of partial 

 application whether cosmical physical forces, light, 

 heat, electricity and the rest, are reducible to the same 

 rule. What then ? It is, I think, scarcely possible to 

 come to an absolute decision upon this point yet. At 

 the same time, without closing my eyes, I cannot help 

 seeing that even now, there is some reason for widening 

 the sphere of the correlation in question until it is co- 

 extensive with that of the physical universe. 



It is difficult to regard the heat of the solar ray as a 

 distinct force : it is less difficult to regard it as a modifi- 

 cation, brought about under particular circumstances, 

 of some other force. In distant space this ray is 

 coldness itself : and the simple fact appears to be that 

 it acquires heat in its transmission through the atmo- 

 sphere chiefly. Indeed, the case is one which agrees 

 very well with the notion that heat may be correlated 

 with the force of gravity, or some other force, and that 

 heat is evolved out of this force by the resistance of the 

 atmosphere just as heat is evolved out of electricity, by 

 the resistance of an insufficient conductor. 



And as with the heat of the solar ray so with the 

 light. This ray is invisible unless it be intercepted in its 

 passage by some material object, and in distant space, 

 for anything that appears to the contrary, it may be as 

 lightless as it would seem to be heatless, the force of 



