BRIDGMAN. — A DETERMINATION OF COMPRESSIBILITIES. 267 



also observing the change of length by an electrical contact device, 

 finds 3.9 X 10 -7 . The iron used by Richards was commercial wrought 

 iron, chemical analysis of which is not given. The mild Bessemer 

 steel used in this investigation is usually as free from carbon as wrought 

 iron, and is very much more likely to be homogeneous. The absence 

 of set is evidence of the closeness of texture, while Richards states that 

 the wrought iron used by him was porous and had to be hammered 

 to give satisfactory results. This possibly may account for some of 

 the difference in the results. 



To get some idea of the effect of varying percentage of carbon, the 

 compressibility of a piece of high carbon (1.25 per cent) annealed tool 

 steel was determined with the same probable error as in the other de- 

 terminations, and was found to be 0.000000525. The discrepancies 

 between Richards' values and the values found in this paper can 

 hardly be explained by impurities of this nature. 



It is to be noted that neither the steel nor the aluminum shows any 

 tendency to become decreasingly compressible at higher pressures, in 

 analogy with the behavior of more compressible substances, particu- 

 larly liquids. In fact, as will be seen from an inspection of either the 

 curves or the table, the aluminum shows a distinct though slight 

 tendency to become more compressible at higher pressures. However, 

 it did not seem that this single example would justify the conclusion 

 that this paradoxical behavior was due to anything except errors of 

 observation, and accordingly the coefficient was calculated by least 

 squares on the assumption that it was constant. 



The second modification of the above method for measuring linear 

 compressibility consists in comparing the change of length of a tube 

 of the substance in question with the simultaneous change of length 

 of a piece of steel, both the substance and the steel suffering uniform 

 contraction by the hydrostatic pressure over the whole exterior sur- 

 face. From the relative change of length the absolute linear com- 

 pressibility may be found if the linear compressibility of the compari- 

 son piece of steel is known. This latter may be found by the first 

 method given above. 



The apparatus with which the relative change of length of the tube 

 (in this case of glass) and the steel were determined is shown in Figure 6. 

 The glass tube C was kept pressed against the bottom B of the cylin- 

 drical hole in the steel cylinder A, by the spring at G, through the me- 

 dium of the tie rod H and the nuts E and F. A split brass ring D slides 

 on the glass tube easily, but tightly enough to remain securely in posi- 

 tion under moderate jarring. Fine scratches were made on the steel 

 at I and the flange of the brass ring. The whole combination was 



