A MODEL OF NATURE. 177 



and must, to that extent, at all events, diii'er f rom matter in hulk as it 

 is directly presented to the senses. 



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

 have a limited mimher of pro[)erti(\s (different, it may ))e, from those 

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

 to a considerable extent, be explained as consequences of the constitu- 

 tion of these separate pai'ts; we shall have succeeded in establishing-, 

 with regard to quantitative properties, a simplitication similar to that 

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

 The many will have been reduced to tin* few. 



The proofs of the physical reality of the entities discovered In' means 

 of the two analyses nuist necessarily be diti'erent. 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 ((utdities 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 groiuid for lielicving that all matter is made 

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

 edge of the constitution or pi'operties which those parts possess. 



THE COAKSE-(iKAINEDNESS OF MATTER. 



Matter in bulk appears to 1)6 continuous. Such substances as water 

 or air appear to the ordinary observ(>r to be perfectly uniform in all 

 their properties and (jualities, in all their parts. 



'I'he hasty conclusion that these bodies are really uniform is, never- 

 theless, unthinka])le. 



In the first place the phenomena of diffusion afford conclusive proof 

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

 conunotion. I need not recai)itulate the familiar evidence to prove 

 that gases and many li<|uids when placed in communication interpene- 

 tiate 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 ]>y licjuids and gases alone, nor by solids at high tempera- 

 tures onh'. Sir W. Ro])erts-Austen has placed pieces of gold and 

 lead in contact at a temperature of 18"^^ C. After four 3'ears the gold 

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

 metals united, but, on analysis, ai)precia})le 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 

 fi-om the surface gold had penetrated into the lead to th(> extent of 

 SM lOUl 12 



