206 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
all their substance. Agitation sufficient to produce even the Brown- 
ian movement might easily exist in such a system. 
Clearly there is nothing impossible or obviously contradictory to 
experimental knowledge in the notion that atoms are compressible; 
indeed, the old idea of small, hard particles far apart is really more 
arbitrary and hypothetical than the new conception. The obvious 
simplicity of the latter is rather in its favor than otherwise, as in 
Dalton’s atomic theory. In general, the more simply an hypothesis 
interprets the phenomena of nature, the more useful the hypothesis 
is likely to be, provided, of course, that the interpretation is adequate. 
The modern philosophy of pragmatism is a good guide in such matters; 
a theory not obviously illogical should be judged by its usefulness. 
Let us then test the new hypothesis by applying it to other aspects 
of physical chemistry. 
If pressure produces a change in the sizes of the atoms and mole- 
cules themselves, may not the actual volumes of liquids and solids be 
used as a guide to the unknown internal pressures within them ? 
Can not we thus discover whether or not chemical affinity exerts 
pressure in its action? To follow this clue, the simplest possible case 
was chosen at first, namely, the comparison of the contractions taking 
place on combining several elements in succession with a single very 
compressible one. The changes of volume occurring during the for- 
mation of oxides were first computed; later, chlorides and bromides 
were studied. According to the theory of compressible atoms, we 
should expect to find greater contraction in cases of greater affinity. 
The diagram (fig. 2), which depicts typical data concerning certain 
nearly related chlorides, strongly supports this inference. One of 
these lines shows the total change of volume which: occurs when a 
gram-molecule of chlorine combines with the equivalent weight of 
metal; the other gives the heat evolved during combination. The 
lines show distinct parallelism; that is to say, reactions evolving 
much heat manifest great contraction. In cases of this kind the 
heat of reaction is usually not very different from the change of free- 
energy ; therefore we may infer that greater affinity is associated with 
greater contraction; and it is but a small leap in the dark to guess 
that the change of volume is caused by the pressure of affinity. 
Since chemical attraction holds two elements firmly together, why 
should it not exert pressure? And if it exerts pressure, why should 
not the volume of the system be diminished by this pressure ? 
This interpretation is not wholly new. Faraday’s great teacher, 
Davy,’ pointed out for the first time a similar fact, namely, that the 
contraction which takes place on forming the oxide of potassium is 
1 Richards, Proceedings American Academy, 1902, vol. 37, p. 399; also, especially, Richards and Jones, 
Journal American Chemical Society, 1909, vol. 31, p. 188. 
8 Humphry Davy, Collected Works, 1840, vol. 5, p. 133 (footnote). 
