September 20, 1901.] 



SCIENCE. 



433 



surface gold had penetrated into the lead 

 to the extent of 1 oz. G dwts. per ton, an 

 amount which could have been profitably 

 extracted . 



Whether it is or is not possible to devise 

 any other intelligible account of the cause 

 of such phenomena, it is certain that a 

 simple and adequate explanation is found 

 in the hypothesis that matter consists of 

 discrete parts in a state of motion, which 

 can penetrate into the spaces between the 

 corresponding parts of the surrounding 

 bodies. 



The hypothesis thus framed is also the 

 one which affords a rational explanation of 

 other simple and well-known facts. If mat- 

 ter is regarded as a continuous medium the 

 phenomena of expansion are unintelligible. 

 There is, apparently, no limit to the ex- 

 pansion of matter, or, to fix our attention 

 on one kind of matter, let us say to the ex- 

 pansion of a gas ; but it is inconceivable 

 that a continuous material which fills or is 

 present in every part of a given space could 

 also be present in every part of a space a 

 million times as great. Such a statement 

 might be made of a mathematical abstrac- 

 tion ; it cannot be true of any real substance 

 or thing. If, however, matter consists of 

 discrete particles, separated from each other 

 either by empty space or by something dif- 

 ferent from themselves, we can at once un- 

 derstand that expansion and contraction 

 may be nothing more than the mutual 

 separation or approach of these particles. 



Again, no clear mental picture can be 

 formed of the phenomena of heat unless we 

 suppose that heat is a mode of motion. In 

 the words of Eumford, " it is extremely 

 difficult, if not quite impossible, to form 

 any distinct idea of anything capable of 

 being excited and communicated in the 

 manner the heat was excited and communi- 

 cated in [his] experiment [on friction] ex- 

 cept it be motion." * And if heat be mo- 



*PhU. Trans,, 1798, p. 99. 



tion, there can be no doubt that ib is the 

 fundamental particles of matter which are 

 moving. For the motion is not visible, is 

 not motion of the body as a whole, while 

 diffusion, which is a movement of matter, 

 goes on more quickly as the temperature 

 rises, thereby proving that the internal mo- 

 tions have become more rapid, which is ex- 

 actly the result which would follow if these 

 were the movements which constitute sen- 

 sible heat. 



Combining, then, the phenomena of dif- 

 fusion, expansion and heat, it is not too 

 much to say that no hypotheses which 

 make them intelligible have ever been 

 framed other than those which are at the 

 basis of the atomic theory. 



Many other considerations also point to 

 the same conclusion. Many years ago 

 Lord Kelvin gave independent arguments, 

 based on the properties of gases, on the 

 constitutions of the surfaces of liquids, and 

 on the electric properties of metals, all of 

 which indicate that matter is, to use his 

 own phrase, coarse-grained — that it is not 

 identical in constitution throughout, but 

 that adjacent minute parts are distinguish- 

 able from each other by being either of dif- 

 ferent natures or in different states. 



And here it is necessary to insist that all 

 these fundamental proofs are independent 

 of the nature of the particles or granules 

 into which matter must be divided. 



The particles, for instance, need not be 

 different in kind from the medium which 

 surrounds and separates them. It would 

 suffice if they were what may be called 

 singular parts of the medium itself, differ- 

 ing from the rest only in some peculiar 

 state of internal motion or of distortion, 

 or by being in some other way earmarked 

 as distinct individuals. The view that the 

 constitution of matter is atomic may and 

 does receive support from theories in which 

 definite assumptions are made as to the 

 constitution of the atoms ; but when, as is 



