COMPRESSIBILITY OF METALS. 107 



A new method of measuring compressibility has been developed for 

 this work, which has very considerable advantages, both of speed and 

 accuracy, over previous methods. As compared with previous 

 methods, the accuracy of this is much increased by the fact that all 

 the corrections are very much less. The method essentially involves 

 the measurement of the difference of linear compressibility between 

 the substance in question and pure iron. If the absolute linear com- 

 pressibility of iron is known, we have at once the linear compressi- 

 bility of the substance in a definite direction, and if equal compressi- 

 bility in all directions is assumed, the true volume compressibility 

 may be computed. It is to be noticed that over the pressure range 

 used here, the compressibility cannot be found by simple multiplica- 

 tion by the factor three, but a correction has to be applied, which in 

 some cases may rise to the order of ten per cent. 



The assumption that the substance is equally compressible in all 

 directions is applicable only when the material is amorphous or when 

 it belongs to that one of the crystalline systems enjoying this property, 

 namely the cubic. This assumption is true for the majority of metals, 

 but it is definitely not true for several, which may have very materially 

 different compressibilities in different directions. For these substances 

 it is necessary to measure the linear compressibility in several directions 

 in order to get the true cubic compressibility. This is at once a dis- 

 advantage and an advantage; the determination of the average cubic 

 compressibility becomes more complicated than by the methods 

 employed by Richards or A. W. J., but on the other hand the average 

 cubic compressibility is not a datum of much significance in these 

 cases, whereas it is possible by the use of the present method to 

 obtain a complete description of the behavior under pressure. For 

 substances of this type it is necessary to have single crystals. The 

 growth of such crystals involves a technique in itself; in this paper 

 only the beginnings of the attack on this subject are made. I have 

 shown for a number of non-cubic crystals that there may be very 

 great differences of linear compressibility in different directions, and 

 have made a promising beginning at a method of obtaining metals 

 with a uniform crystalline orientation throughout the entire mass. 

 The further examination of this important question must be left for 

 future work. 



■With regard even to those metals which are known to crystallize in 

 the cubic system it is not safe to assume without some examination 

 that the linear compressibility is the same in every direction, for few 

 masses of metal are unicrystalline, but there are also regions of 



