204 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
actually occupied by the molecules) is changeable, are not the 
molecules themselves compressible ? 4 
The next step in the train of thought is perhaps equally obvious. 
If changes in the bulk of molecules are to be inferred even from 
gases, may not the expansion and contraction of solids and liquids 
afford a much better clue to the relative expansion and contraction 
of these molecules ? x 
Most physical chemists refer all changes in volume to changes in 
the extent of the empty space between the molecules. But are 
there, after all, any ‘such empty spaces in solids and liquids? Solids 
do not behave as if the atoms were far apart within them; porosity 
is often conspicuous by its absence. Take, for instance, the case of 
glass; the careful experiments of Landolt on the conservation of 
weight ? show that glass is highly impermeable to oxygen, nitrogen 
and water for long periods. Such porosity as occurs in rigid, com- 
pact solids usually permits the passage only of substances which 
enter into the chemical structure of the solids themselves. Thus, 
nitrogen can not free itself from imprisonment within hot cupric 
oxide, although oxygen can escape; * again, water can not evaporate 
into even the driest of atmospheres from accidental incarceration 
in crystals lacking water of crystallization.t Palladium, on occluding 
hydrogen, is obliged to expand its bulk in order to make room for 
even this small addition to its substance. The behavior of platinum, 
nickel, and iron is probably analogous, although less marked.® 
Fused quartz, impermeable when cold, allows of the passage of 
helium and hydrogen at high temperatures;® but most other gases 
seem to be refused admission and 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 ordinary 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 
1 Van der Waals speaks cautiously, but with some conviction, as to the probable compressibility of the 
molecules on p. 283 of the paper cited above. 
2H. Landolt, Ueber die Erhaltung der Masse bei chem. Umwandlungen, Abhandlung der konigl. 
preuss. Akad. der Wissenschaften, 1910. 
3 Richards, Zeitsch. anorg. Chem., 1892, vol. 1, p. 196; Proceedings American Academy, 1893, vol. 28, p. 200; 
ibid, 1898, vol. 38, p. 399. 
4 Baker and Adlam, Journal Chemical Society Transactions, 1911, vol. 99, p. 507. 
5 Richards and Behr, Publications Carnegie Institution, 1906, No. 61. 
6 Jacquerod and Perrot, Compt. rend., 1907, vol. 144, p. 135. 
7 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, vol. 89, p. 1675; 1907, vol. 91, p 1150; 1908, vol. 93, 
p. 1528; 1910, vol. 97, p. 2308.) 
