7 io THE POPULAR SCIENCE MONTHLY. 



gate volumes of the liquids mixed. But, waiving this, as well as the 

 phenomena which emerge in the processes of solution and chemical ac- 

 tion, it must be said that experience does not in any manner vouch 

 for the impenetrability of matter as such in all its states of aggrega- 

 tion. When gases are subjected to pressure, the result is simply an 

 increase of the expansive force in proportion to the pressure exerted, 

 according to the law of Boyle and Mariotte (the modifications of and 

 apparent exceptions to which, as exhibited in the experimental results 

 obtained by Regnault and others, need not here be stated, because 

 they do not affect the argument). A definite experimental limit is 

 reached in the case of those gases only in which the pressure produces 

 liquefaction or solidification. The most significant phenomenon, how- 

 ever, which experience contributes to the testimony on this subject is 

 the diffusion of gases. Whenever two or more gases which do not act 

 upon each other chemically are introduced into f a given space, each gas 

 diffuses itself in this space as though it were alone present there ; or, 

 as Dalton, the reputed father of the modern atomic theory, expresses 

 it, "Gases are mutually passive, and pass into each other as into 

 vacua." 



Whatever reality may correspond to the notion of the impenetra- 

 bility of matter, this impenetrability is not, in the sense of the atom- 

 ists, a datum of experience. 



Upon the whole, it would seem that the validity of the first propo- 

 sition of the atomic theory is not sustained by the facts. Even if the 

 assumed unchangeability of the supposed ultimate constituent particles 

 of matter presented itself, upon its own showing, as more than a bare 

 reproduction of an obseiwed fact in the form of an hypothesis, and 

 could be dignified with the name of a generalization or of a theory, 

 it would still be obnoxious to the criticism that it is a generalization 

 from facts crudely observed and imperfectly apprehended. 



In this connection it may be observed that the atomic theory has 

 become next to valueless as an explanation of the impenetrability 

 of matter, since it has been pressed into the service of the modulatory 

 theory of light, heat, etc., and assumed the form in which it is now 

 held by the majority of physicists, as we shall presently see. Ac- 

 cording to this form of the theory, the atoms are either mere points, 

 wholly without extension, or their dimensions are infinitely small as 

 compared with the distances between them, whatever be the state 

 of aggregation of the substances into which they enter. In this view 

 the resistance which a body, i. e., a system of atoms, offers to the in- 

 trusion of another body is due, not to the rigidity or unchangeability 

 of volume of the individual atoms, but to the relation between the 

 attractive and repulsive forces with which they are supposed to be 

 endowed. There are physicists holding this view who are of opinion 

 that the atomic constitution of matter is consistent with its impene- 

 trability among them M. Cauchy, who, in his Sept Lecons de Phy- 



