180 BRIDGMAN. 



The procedure in making the calculations outlined in the paragraph 

 above was applied independently at the two temperatures 30° and 75°. 

 Departure from linearity was in most cases so small that it was useless 

 to attempt to try to get the temperature coefficient of the departure. 

 Accordingly in the calculations the mean of the departures found 

 independently at the two temperatures was usually used as that most 

 probably accurate. 



The behavior of the metals under pressure gives in many cases a 

 useful check on their probable homogeneity and therefore on the 

 equality of their compressibility in all directions. If the metal is 

 under no internal strains, the relation between deformation and 

 pressure should be single valued, without hysteresis, and there should 

 be no permanent change of dimensions, even after the first application 

 of pressure. The absence of hysteresis could of course be checked 

 by the readings themselves, but the freedom from set on the initial 

 application of pressure was not so easy to determine, because there 

 were sometimes slight initial irregularities in the apparatus itself which 

 were smoothed out by the initial application of pressure. In all cases 

 the metal was subjected to a preliminary application of pressure over 

 the entire range before readings were begun, and the change of zero 

 produced by this preliminary application was also recorded. This 

 should give an upper limit to any actual change of dimensions pro- 

 duced in the specimen by pressure. If the substance is carefully 

 prepared there should be no change, and tins was in almost every case 

 the fact. It was possible to make castings of lead, for example, that 

 showed no perceptible permanent change of length after the initial 

 application of 12000 kg. In one or two cases, however, there were 

 comparatively large changes, and these were usually accompanied by 

 hysteresis on subsequent applications of pressure, showing internal 

 strains. These cases will be described in detail later. 



There follows now the detailed presentation of data. First are 

 given those metals crystallizing in the cubic system. There are 

 included here a few metals whose structure has not yet been deter- 

 mined, since the chances are that any metal selected at random is 

 cubic. The metals are arranged in order of compressibility, beginning 

 with tungsten, the least compressible. After this are given data for 

 several metals not crystallizing in the cubic system. The work on 

 these latter metals must be extended. The present data are com- 

 petent to give only an idea of the amount of variation to be expected 

 in the compressibility in different directions. 



