432 



SCIENCE. 



[N. S. Vol. XIV. No. 351. 



case of Saturn's rings, clouds, smoke, and a 

 number of similar instances. The believer 

 in the atomic theory asserts that matter 

 exists in a particular state ; that it consists 

 of parts which are separate and distinct the 

 one from the other, and as such are capable 

 of independent movements. 



Up to this point no question arises as to 

 whether the separate parts are, like grains 

 of sand, mere fragments of matter; or 

 whether, though they are the bricks of 

 which matter is built, they have, as indi- 

 viduals, properties different from those of 

 masses of matter large enough to be directly 

 perceived. If they are mere fragments of 

 ordinary matter, they cannot be used as aids 

 in explaining those qualities of matter 

 which they themselves share. 



We cannot explain things by things them- 

 selves. If it be true that the properties of 

 matter are the product of an underlying 

 machinery, that machinery cannot itself 

 have'the properties which it produces, and 

 must, to that extent at all events, differ 

 from matter in bulk as it is directly pre- 

 sented 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, be explained as 

 consequences of the constitution 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 re- 

 gard to varieties of matter. The many 

 will have been reduced to the 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 ele- 

 mentary constituents into which he has re- 

 solved a compound mass. No physicist or 

 chemist can produce a single atom separated 



from all its fellows, and show that it pos- 

 sesses the elementary 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 hypoth- 

 esis that they possess that constitution ex- 

 plains. 



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 up of discrete parts ; and 

 secondly, whether we can have any knowl- 

 edge of the constitution or properties which 

 those parts possess. 



THE COARSB-GRAINEDNESS OF MATTER, 



Matter in bulk appears to be continuous. . 

 Such substances as water or air appear to 

 the ordinary observer to be perfectly uni- 

 form in all their properties and qualities, 

 in all their parts. 



The hasty conclusion that these bodies are 

 really uniform is, nevertheless, unthinkable. 



In the first place the phenomena of dif- 

 fusion 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 interpenetrate or diffuse 

 into each other ; or that air, in contact with 

 a surface of water, gradually becomes laden 

 with water vapor, while the atmospheric 

 gases in turn mingle with the water. Such 

 phenomena are not exhibited by liquids and 

 gases alone, nor by solids at high temper- 

 atures only. Sir W. Eoberts-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 quan- 

 tities of the gold were detected even at a 

 distance of more than 5 millimeters from 

 the common surface, while within a distance 

 of three-quarters of a millimeter from the 



