PRIMARY CONCEPTS OF MODERN SCIENCE. 709 



Masses find their true and only measure in the action of forces, and 

 the quantitative persistence of the effect of this action is the simple 

 and accurate expression of the fact which is ordinarily described as 

 the indestructibility of matter. It is obvious that this persistence is 

 in no sense explained or accounted for by the atomic hypothesis. It 

 may be that such persistence is an attribute of the minute, insensible 

 particles which are supposed to constitute matter, as well as of sen- 

 sible masses ; but, surely, the hypothetical recurrence of a fact in the 

 atom is no explanation of the actual occurrence of the same fact in 

 the conglomerate mass. Whatever mystery is involved in the phe- 

 nomenon is as great in the case of the atom as in that of a solar or 

 planetary sphere. Breaking a magnet into fragments, and showing 

 that each fragment is endowed with the magnetic polarity of the in- 

 teger magnet, is no explanation of the phenomenon of magnetism. A 

 phenomenon is not explained by being dwarfed. A fact is not trans- 

 formed into a theory by being looked at through an inverted telescope. 

 The hypothesis of ultimate indestructible atoms is not a necessary im- 

 plication of the persistence of weight, and can at best account for the 

 indestructibility of matter if it can be shown that there is an absolute 

 limit to the compressibility of matter in other words, that there is 

 an absolutely least volume for every determinate mass. This brings 

 us to the consideration of that general property of matter Avhich prob- 

 ably, in the minds of most men, most urgently requires the assump- 

 tion of atoms its impenetrability. 



" Two bodies cannot occupy the same space " such is the familiar 

 statement of the fact in question. Like the indestructibility of matter, 

 it is claimed to be a datum of experience. " Corpora omnia impene- 

 trdbilia esse" says Sir Isaac Newton (Phil. Nat. Princ. Math., lib. 

 iii., reg. 3), " non ratione sed sensu colligimus." Let us see in what 

 sense and to what extent this claim is legitimate. 



The proposition, according to which a space occupied by one body 

 cannot be occupied by another, implies the assumption that space is 

 an absolute, self-measuring entity an assumption which I may have 

 occasion to examine hereafter and the further assumption that there 

 is a least space which a given body will absolutely fill so as to exclude 

 any other body. A verification of this proposition by experience, 

 therefore, must amount to proof that there is an absolute limit to the 

 compressibility of all matter whatsoever. Now, does experience au- 

 thorize us to assign such a limit ? Assuredly not. It is true that in 

 the case of solids and liquids there are practical limits beyond which 

 compression by the mechanical means at our command is impossible ; 

 but even here we are met by the fact that the volumes of fluids, which 

 effectually resist all efforts at further reduction by external pressure, 

 are readily reduced by mere mixture. Thus, sulphuric acid and water 

 at ordinary temperatures do not sensibly yield to pressure ; but, when 

 they are mixed, the resulting volume is materially less than the aggre- 



