218 BRIDGMAN. 



have no chemical analysis. This was prepared as a unicrystalline 

 casting in the manner already described for cadmium. The crystal- 

 line figures were very prominent in this casting, and show the orien- 

 tation of the crystal to be the same throughout the entire casting. The 

 appearance of the casting is somewhat tree-like, the axis of the tree is 

 inclined at an angle of about 15° to the axis of the casting. 



The specimen was mounted as a compression specimen (length 16 

 cm.) in the apparatus for direct measurement. Only a few measure- 

 ments were made on it, because it was at once evident that the behav- 

 ior is such as to be worthy of much more detailed study, which must 

 be put off until a systematic investigation can be made of the variation 

 of compressibility with direction of a large number of crystals. The 

 measurements, as far as they went, were perfectly regular. There 

 was very little permanent set on the first application of pressure, and 

 the relation between distortion and pressure was sensibly linear. 

 The startling result found for this substance was that its compressi- 

 bility along the axis of the casting is negative; that is, that it lengthens 

 along the axis when subjected to a uniform hydrostatic pressure 

 all over. Its average linear compressibility in this direction was 

 -2.36 X 10- 7 . 



Paradoxical as this behavior appears, there is nothing inherently 

 impossible about it, and something analogous has been observed by 

 Voigt in the negative Poisson's ratio in certain directions in pyrites. 

 (This remark is merely by way of analogy; the linear compressi- 

 bility of pyrites is not negative in any direction.) 



Discussion of Results. 



Although the experiments just described give the compressibility 

 of some substances not measured before, and also I believe give the 

 compressibilities with greater accuracy than hitherto attained, the 

 principle interest in the measurements is in the variations of com- 

 pressibility with temperature and pressure. The variation of thermal 

 expansion with pressure is also given by the experiments, because 

 of course it is a mathematical identity that the pressure coefficient 

 of thermal expansion is the same as the temperature coefficient of 

 compressibility. 



The results of these experiments are collected in Table IV. In the 

 first column is given the name and description of the substance, in the 

 second column the initial compressibility at 30° multiplied by 10 7 , 



