542 



SCIENCE 



[N. S. Vol. XXXIV. No. 878- 



very many solid substances appear to act 

 as effective barriers to the passage of even 

 hydrogen and helium, especially when cold. 

 In these cases, as in so many others, the so- 

 called "sphere of influence" of the atom is 

 the actual boundary by which we know the 

 atom and measure its behavior.^' Why not 

 call this the actual bulk of the atom? 



From another point of view, the ordi- 

 nary conception of a solid has always 

 seemed to me little short of an absurdity. 

 A gas may very properly be imagined with 

 moving particles far apart, but what could 

 give the rigidity of steel to such an un- 

 stable structure? The most reasonable 

 conclusion, from all the evidence taken to- 

 gether, seems to be that the interstices be- 

 tween atoms in solids and liquids must 

 usually be small even in proportion to the 

 size of the atoms themselves, if, indeed, 

 there are any interstices at all. 



Very direct and convincing evidence of 

 another sort is at hand. The idea that 

 atoms may be compressible receives striking 

 confirmation from a recent interesting in- 

 vestigation of Griineisen^^ concerning the 

 small effect of low temperatures on the 

 compressibility of metals. The average 

 compressibility of aluminium, iron, copper, 

 silver and platinum falls off only seven per 

 cent, between the temperature of the room 

 and that of liquid air. Extrapolation of 

 the curves indicates that at the absolute 

 zero very little further diminution should 



" Since these ideas were first advanced, Barlow 

 and Pope have brought forward much interesting 

 evidence concerning the significance of the volumes 

 of solids and liquids, which supports the idea that 

 the atoms are closely in contact with one another 

 {Trans., 1906, 89, p. 1675; 1907, 91, p. 1150; 1908, 

 93, p. 1528; 1910, 97, p. 2308). 



"''E. Gruneisen, Ann. Physilc, 1910 (IV.), 33, 

 p. 1239. The relative values for the compressibili- 

 ties recorded in this investigation are doubtless 

 trustworthy, although the absolute magnitudes are 

 somewhat uncertain because they depend on the 

 rather inadequate theory of elasticity. 



occur. As far as we can guess, therefore, 

 the hard metals are almost as compressible 

 at the absolute zero as at room tempera- 

 tures. But at the absolute zero all heat- 

 vibration is supposed to stop; hence this 

 remaining compressibility must needs be 

 ascribed to the atoms themselves. 



If the atoms are compressible, all mathe- 

 matical reasoning which assumes them to- 

 be incompressible rests upon a false basis. 

 The kinetic theory of gases remains un- 

 molested by these considerations, except as 

 they indicate the changeability of b in the 

 equation of van der Waals, but the new 

 views affect seriously the application of this 

 equation to solids and liquids. 



Let us proceed to trace a few of the out- 

 comes of our hypothesis. If atoms may 

 really be packed closely together, the vol- 

 umes of solids and liquids should afford 

 valuable knowledge concerning the relative 

 spaces occupied by the atoms themselves 

 under varying conditions. The densities 

 of solids and liquids then assume a signifi- 

 cance far more interesting to the chemical 

 philosopher than before, because they have 

 a more definite connection with the funda- 

 mental nature of things. 



An apparent objection at once suggests 

 itself; if the particles in condensed ma- 

 terial are really touching one another, how 

 can we account for heat within the ma- 

 terial? Would such closely packed atoms 

 be able to vibrate ? 



The theory of compressible atoms sup- 

 plies as one of its own corollaries the imme- 

 diate answer to this question. If atoms are 

 compressible throughout their whole sub- 

 stance, they may contract and expand, or 

 vibrate within themselves, even when their 

 surfaces are prevented from moving by be- 

 ing closely packed together. It is thus- 

 possible to conceive of a vibrational effect, 

 even in contiguous atoms, provided we can 

 conceive of these atoms as being elastic 



