178 A MODEL OF NATURE, 



1 ouiK'e 6 pennj'weights per ton, an amount which could have been 

 profitably extracted. 



Whether it is or i.s not possible to devise an}' other intelligible 

 account of the cause of such phenomena, it is certain that a simple and 

 adetpiate 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 

 exphmation of other simple and well-known facts. If matter is 

 regarded as a continuous medium the phenomena of expansion are 

 iniintelligible. There is, apparentl}^, no limit to the expansion of 

 matter, or, to tix our attention on one kind of matter, let us say to the 

 expansion of 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 ever}^ part of a space a million times as great. Such a state- 

 ment might be made of a mathematical abstraction; it can not be true 

 of an}^ real substance or thing. If, however, matter consists of dis- 

 crete particles, separated from each other either b}^ empty space or 

 by something different from themselves, we can at once understand 

 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 Rumford, " it is extremely difficult, if not quite impossible, 

 to form an}' distinct idea of anything capable of being excited and 

 comnumicated in the manner the heat was excited and communicated 

 in [his] experiment [on friction] except it be motion.'"" And if heat 

 be motion, there can be no doubt that it is the fundamental particles 

 of matter which are moving. For the motion is not visi))le, is not 

 motion of the body as a whole, while diffusion, which is a movement 

 of matter, goes on more quickl}^ as the temperature rises, thereby 

 proving that the internal motions have become more rapid, which is 

 exactly the result which would follow if these were the movements 

 which constitute sensible heat. 



Combining, then, the phenomena of diffusion, expansion, and heat, 

 it is not too much to say that no hypotheses which make them intelli- 

 gible 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 

 yeai's 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 



Thil. TraiiH., 1789, p. 99. 



