476 



Sc ien tific In telligen ce. 



of the rod, when the pressure is relieved, that change of position 

 is composed of two motions, the expansion of the part of the rod 

 which lies between the right-hand extremity and the point in it 

 whose motion with respect to the steel carrying tube is nil, along 

 with the proper motion of that point. Similarly, when we meas- 

 ure the change of position of the left-hand end, it also is com- 

 posed of two parts, the expansion of the part of the rod which 

 lies between the left-hand extremity and the same point in the 

 length of the rod where its motion with respect to the steel tube 

 is nil, along with the proper motion of that point. But at the 

 left-hand end the motion of expansion is to the left, and at the 

 right-hand end it is to the right, while the proper motion of the 

 position of the common point on the rod and on the tube is 

 always in one direction, and in this case, to the left. Therefore 

 the distance measured in the right-hand microscope is the expan- 

 sion of the portion of the rod which lies to the right of the point 

 on it which is motionless relatively to the tube minus the proper 

 motion of this point : and the distance measured at the left-hand 

 end is the expansion of the remainder of the rod plus the proper 

 motion of the common point. Consequently the algebraic sum of 

 the two motions measured is the expansion of the rod under the 

 relief of pressure." 



The following table gives a summary of the results obtained 

 which are stated in full in six tables preceding. This final table 

 is as follows : 







Year. 



Atomic 

 weight. 



Density. 



Compressibility. 



Substance. 



Linear. 



Cubic. 



Platinum 



Gold 



Copper . . 



Aluminium 



Magnesium 



1904 



u 

 a 



t( 



a 



194 



197 

 63 

 27 

 24 



21-5 



19'3 

 8-9 

 2-6 

 1-75 



* 13-6 



2-968 

 2-494 



0-1835 



0-260 



0-288 



0-558 



1-054 



0-5505 



0-780 



0-864 



0-1674 



3-162 



Mercury 



Glass, flint 







" German .. 



1875 



1880 

 1904 



200 



1-33 



0-973 



1-02 



0-846 



3-99 

 2-92 

 3-06 

 2-54 



"In this summary the compressibilities of English flint glass and 

 of the glass of which ordinary German tubing is made as well as 

 that of mercury have been included for purposes of comparison. 

 The compressibility of mercury rests upon a large number of 

 observations made in the ' Challenger,' by which its apparent 

 cubic compressibility was found to be 1*5 per million per atmos- 

 phere." 



The author calls attention to the relation existing between 

 compressibility and density, and remarks that a field for interest- 

 ing research is here opened in which the method is capable of 

 great refinement. 



