208 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
stances concerned. The greater the compressibility, the greater 
should be the change of volume caused by a given pressure of affinity. 
Before any definite conclusion can be drawn, the differences in com- 
pressibility must be taken into account. 
These thoughts led to the measuring of the compressibilities of a 
large number of elements and simple compounds. The previously 
employed methods for solids and liquids being unsatisfactory, a new 
and highly satisfactory method was devised for the work done at 
Harvard. Pure mercury is compressed in a suitable tube, measuring 
both pressure and change of volume, and then most of the mercury 
is displaced by the substance to be studied, again noting the rela- 
tionship of pressure to volume. The difference between the com- 
pressibility of mercury and that of the substance is then easily cal- 
culated. Obviously, in such a method as this, the compressibility 
of the apparatus itself is eliminated. The relation of volume to 
pressure is easily determined by causing the mercury meniscus to 
make electrical contact with a very fine platinum point in a tube of 
narrow diameter, adding weighed globules of mercury, and noting 
the corresponding pressures.!. Time forbids the description of the 
details of the procedure. 
The. compressibilities of 35 elements and many simple compounds 
were studied by this method with sufficient care to leave no doubt 
as to their relative values. It became at once manifest that the 
formation of a compound of a compressible element was attended 
with greater decrease of volume than the formation of a similar com- 
pound of a less compressible element, other things being equal.? 
This is just what the theory leads us to expect, and is a fact inexplica- 
ble by any other hypothesis as yet known to me. 
Another essential aspect of the theory of compressible atoms is 
that which concerns cohesion.’ If the pressure of chemical affinity 
causes atomic compression, may not the pressure of cohesive affinity 
also have the same effect? Traube suggested this possibility, but 
looked at the whole question from a different point of view.* The 
affinity which prevents solids and liquids from vaporizing is generally 
admitted to produce great internal pressure; must it not tend to com- 
press the molecules into smaller space? Molecules with high cohesive 
affinity (those of substances hard to volatilize) should be much com- 
1 Richards, in collaboration with Stull, Bonnet, Brink, Mathews, Jones, Speyers, Publication Carnegie 
Institute of Washington, Nos. 7 and 76; Journal American Chemical Society, 1904, vol. 26, p. 399; 1909, vol. 
31, p. 154; Zeitsch. physikal. Chem., 1904, vol. 49, p. 1; 1907, vol. 61, p. 77. 
2 Richards, Proceedings American Academy, 1904, vol. 39, p. 581. 
Sea 
4 yes Traube Ann. Physik., 1897, (iii), vol. 61, p. 383; 1901 (iv.), vol. 5; p. 548; 1902, vol. 8, p. 
267; 1907, vol. 22, p. 519; Zeitsch. physikal. Chem., 1910, vol. 68, p. 289; also Walden, Zeitsch. physikal. 
Chem., 1909, vol. 66, p. 385. Their interpretation depends largely on the application of van der Waals’s 
equation and the complicating assumption of a covolume; however, Walden’s very recent paper presents a 
number of interesting and important relations concerning internal pressure, which seem to demand the 
assumption of atomi¢ compressibility for their explanation. 
