120 Mr. S. T. Preston on the Application of the 



evidently one molecule cannot readily penetrate into the inter- 

 stices of another. On the other hand, the minute particles of 

 the gravific medium pass through them with perfect freedom ; 

 and though these interstices are so small, they are on the other 

 hand so numerous (on account of the number of the molecules) 

 that their total sum may represent a relatively very large va- 

 cant space. Under these conditions matter may be practically 

 solid or continuous, because impenetrable by the finest por- 

 tions (molecules) of other matter, and yet possess any desired 

 degree of openness. 



8. We would add a few remarks here in regard to the logical 

 necessity of seeking a cause for gravitation. To do so is, 

 as it seems to us, simply to look for an explanation of a natural 

 phenomenon consistent with reason. One sometimes comes 

 across the remark that the effect is an ultimate one, incapable 

 of explanation. But then the physical investigator does not 

 readily surrender the right of using his reason ; or we really 

 have no power to assume that physical effects are brought 

 about in a way incapable of appreciation by the reason. The 

 most eminent minds have admittedly been in favour of an ex- 

 planation. This was so (as is known) with Newton and 

 Faraday. Count Bumford says, " Nobody surely in his sober 

 senses has ever pretended to understand the mechanism of 

 gravitation." Physical effects are generally admitted to be 

 fundamentally effects of motion (however diverse they may 

 be). The one fundamental cause, therefore, to get an insight 

 into in physical science, is the cause of the development of 

 motion. If we made an exception to this in any case (or 

 assumed the motion developed was an ultimate fact incapable 

 of explanation), then this would be pursuing a course which, 

 if carried out in its entirety would leave nothing to be explained 

 at all ; for it should be observed that the development of mo- 

 tion is in principle the one physical effect that requires expla- 

 nation (from the fact that all physical effects are effects of 

 motion). This inference surely deserves a mature realization. In 

 the case of gravity we observe a motion of approach developed 

 in two masses. Here, therefore, we have an instance of the 

 one fundamental fact for which in principle an explanation is 

 required. We require an insight into the cause of the deve- 

 lopment of this motion in the two masses. We want something 

 more than merely to observe the fact of the motion ; we want 

 (among other things) to understand why the energy of the 

 motion developed has the particular intensity observed — also 

 to account for the remarkable fact that the intensity diminishes 

 in the complex ratio of the square of the distance, and not in 

 some other ratio. Surely if any thing requires an explanation, 



