A MODEL OF NATURE. 177 



and must, to that extent, at all events, differ from matter in bulk as it 

 is directly presented to the senses. 



If, however, we can succeed in showing that if the separate parts 

 have a limited number of properties (different, it may be, from those 

 of matter in bulk), the many and complicated properties of matter can, 

 to a considerable extent. In' explained as consequences of the constitu- 

 tion of these separate parts; we shall have succeeded in establishing, 

 with regard to quantitative properties, a simplification similar to that 

 which the chemist has established with regard to varieties of matter. 

 The many will have been reduced to tin 1 few. 



The proofs of the physical reality of the entities discovered by means 

 of the two analyses must necessarily be different. The chemist can 

 actually produce the elementary constituents into which he has resolved 

 a compound mass. No physicist or chemist can produce a single atom 

 separated from all its fellows and show that it possesses the elemen- 

 tary qualities he assigns to it. The cogency of the evidence for any 

 suggested constitution of atoms must vary with the number of facts 

 which the hypothesis that they possess that constitution explains. 



Let us take, then, two steps in their proper order, and inquire, first, 

 whether there is valid ground for believing that all matter is made 

 tip of discrete parts; and, secondly, whether we can have any knowl- 

 edge of the constitution or properties which those parts possess. 



THE COARSE-GRAINEDNESS OF MATTER. 



Matter in bulk appears to be continuous. Such substances as water 

 or air appear to the ordinary observer to be perfectly uniform in all 

 their properties and qualities, in all their parts. 



The hasty conclusion that these bodies are really uniform is, never- 

 theless, unthinkable. 



In the first place the phenomena of diffusion afford conclusive proof 

 that matter when apparently quiescent is in fact in a state of internal 

 commotion. I need not recapitulate the familiar evidence to prove 

 that gases and many liquids when placed in communication interpene- 

 trate or diffuse into each other; or that air, in contact with a surface 

 of water, gradually becomes laden with water vapor, while the atmos- 

 pheric gases in turn mingle with the water. Such phenomena are not 

 exhibited by liquids and gases alone, nor by solids at high tempera- 

 tures only. Sir W. Roberts- Austen has placed pieces of gold and 

 lead in contact at a temperature of 18° C. After four years the gold 

 had traveled into the lead to such an extent that not only were the two 

 metals united, but, on analysis, appreciable quantities of the gold were 

 detected even at a distance of more than 5 millimeters from the com- 

 mon surface, while within a distance of three-quarters of a millimeter 

 from the surface gold had penetrated into the lead to the extent of 

 sm L901 12 



