18 SirW. Thomson on the Thermoelastic, Thermomagnetic, 



Substances. 



Authority. 



Ratio of lateral shrinking 

 or swelling to longitudinal 

 extension or shortening 

 under the influence of 

 push or pull on ends of 

 a column of the substance. 



Cork 



crystal 



Specimens of 



glass 



A specimen of flint glass 

 Another specimen of flint 



glass , 



A specimen of brass 

 Drawn brass rod . . 



Copper 



Iron 



Steel 



Cast steel 



Vulcanized india-rubber. 



General experience and 

 some accurate measure- 

 ments of diameter of a 

 cork under various de- 

 grees of end-pressure, 

 producing shortenings 

 from small amounts up 

 to as much as -| of the 

 original length 



I-Wertheim 



Everett (1865) 



j Everett (1866) , 



Wertheim , 



Everett (1866) 



W. Thomson 



Clerk Maxwell 



Kirchhoff 



Everett (1866) 



Joule 



•33 



•23 



•34 



•47 



from -40 to -23 



•27 



•29 



•31 

 J" Less than - 5 by an ex- 

 \ ceedingly small amount 



J 



19. Again, some of the relations which have been supposed 

 to exist lead to three principal axes of elasticity. Many 

 natural crystals do certainly exhibit perfect symmetry of form 

 with reference to three rectangular axes, and therefore pro- 

 bably possess all their physical properties symmetrically with 

 reference to those axes ; but as certainly many, and among 

 them some of the best-known, of natural crystals do not exhibit 

 symmetry of form with reference to rectangular axes, and pos- 

 sess the mechanical property of resisting fracture differently in 

 different directions, without symmetry about any three rectan- 

 gular axes — for instance, Iceland spar, which has three planes 

 of greatest brittleness (" cleavage-planes "), inclined at equal 

 angles to one another and to a common axis (the " optic axis " 

 of the crystal). If, as probably must be the case, the elastic 

 properties within the limits of elasticity have correspondence 

 with the mechanical properties on which the brittleness in dif- 

 ferent directions depends, the last-mentioned class of crystals 

 cannot have three principal axes of elasticity at right angles 

 to one another. It will be an interesting inquiry to examine 

 thoroughly the various directional properties of an elastic solid 

 represented by the different coefficients (of which the entire 

 number may of course be reduced from twenty-one to eighteen 

 by a choice of axes), or by various combinations of them. 



