360 JAMES CLERK MAXWELL. [CHAP. XII. 



It is only when we contemplate, not matter in itself, 

 but the form in which it actually exists, that our mind finds 

 something on which it can lay hold. 



That matter, as such, should have certain fundamental 

 properties, that it should exist in space and be capable of 

 motion, that its motion should be persistent, and so on, -are 

 truths which may, for anything we know, be of the kind 

 which metaphysicians call necessary. We may use our 

 knowledge of such truths for purposes of deduction, but we 

 have no data for speculating as to their origin. 



But that there should be exactly so much matter and no 

 more in every molecule of hydrogen is a fact of a very 

 different order. We have here a particular distribution of 

 matter a collocation to use the expression of Dr. Chalmers, 

 of things which we have no difficulty in imagining to have 

 been arranged otherwise. 



The form and dimensions of the orbits of the planets, 

 for instance, are not determined by any law of nature, but 

 depend upon a particular collocation of matter. The same 

 is the case with respect to the size of the earth, from which 

 the standard of what is called the metrical system has been 

 derived. But these astronomical and terrestrial magnitudes 

 are far inferior in scientific importance to that most funda- 

 mental of all standards which forms the base of the mole- 

 cular system. Natural causes, as we know, are at work, 

 which tend to modify, if they do not at length destroy, all 

 the arrangements and dimensions of the earth and the whole 

 solar system. But though in the course of ages catastrophes 

 have occurred and may yet occur in the heavens, though 

 ancient systems may be dissolved and new systems evolved 

 out of their ruins, the molecules out of which these systems 

 are built the foundation-stones of the material universe 

 remain unbroken and unworn. They continue this day as 

 they were created perfect in number and measure and 

 weight ; and from the ineffaceable characters impressed on 

 them we may learn that those aspirations after accuracy in 

 measurement, and justice in action, which we reckon among 

 our noblest attributes as men, are ours because they are 



