60 Lord Kelvin. On the [June 15, 



into either a dextro-cliiral or levo-chiral configuration in the act of 

 forming a crystal. 



2. Certain it is that the crystalline molecule has a chiral configura- 

 tion in every crystal which shows chirality in its crystalline form or 

 which produces right- or left-handed rotation of the plane of polarisa- 

 tion of light passing through it. The magnetic rotation has neither 

 right-handed nor left-handed quality (that is to say, no chirality). 

 This was perfectly understood by Faraday and made clear in his 

 writings, yet even to the present day we frequently find the chiral 

 rotation and the magnetic rotation of the plane of polarised light 

 classed together in a manner against which Faraday's original de- 

 scription of his discovery of the magnetic polarisation contains ample- 

 warning. 



3. These questions, however, of chirality and magnetic rotation do 

 not belong to my present subject, which is merely the forcive* 

 required to keep a crystal homogeneously strained to any infinitesi- 

 mal extent from the condition in which it rests when no force acts 

 upon it from without. In the elements of the mathematical theory 

 of elasticity! we find that this forcive constitutes what is called a 

 homogeneous stress, and is specified completely by six generalised 

 force-components, p lf jp 2 , p 3 , . . . , jp 6 > which are related to six corre- 

 sponding generalised components of strain, s l7 s 2 , $3, . . . . , s 6 > by the- 

 following formulas : 



where w denotes the work required per unit volume to alter any por- 

 tion of the crystal from its natural unstressed and unstrained condi- 

 tion to any condition of infinitesimal homogeneous stress or strain : 



lh = *?,....,p. = *? ................ (2), 



ds l dss 



where , . . . . , denote differential coefficients on the supposition 

 dsi ds 6 



that w is expressed as a homogeneous quadratic function of i, . . . . , s 6 r 



dw dw 



si = -j-, ---- ,e = -= 

 dp l dp 6 



r r 



where ,...., denote differential coefficients on the supposi- 

 dpi dp 6 



tion that w is expressed as a homogeneous quadratic function of 

 Pi, ...... p 6 . 



* This is a word introduced by my brother, the late Professor James Thomson,. 

 to designate any system of forces. 



f ' Phil. Trans.,' April 24, 1856, reprinted in vol. iii, ' Math, and Phys. Papers * 

 (Sir W. Thomson), pp. 84112. 



