On the Elasticity of a Crystal according to Boscovich. 59 



June 15, 1893. 

 The LORD KELVIN, D.C.L., LL.D., President, in the Chair. 



A List of the Presents received was laid on the table, and thanks 

 ordered for them. 



Professor William Tennant Gairdner and Professor James William 

 H. Trail were admitted into the Society. 



The following Papers were read : 



I. '* On the Elasticity of a Crystal according to Boscovich." 

 By the LORD KELVIN, P.R.S. Received June 8, 1893. 



1. A crystal in nature is essentially a homogeneous assemblage of 

 equal and similar molecules, which for brevity I shall call crystalline 

 molecules. The crystalline molecule may be the smallest portion 

 which can be taken from the substance without chemical decomposi- 

 tion, that is to say, it may be the group of atoms kept together by 

 chemical affinity, which constitutes what for brevity I shall call the 

 chemical molecule ; or it may be a group of two, three, or more of 

 these chemical molecules kept together by cohesive force. In a crystal 

 of tartaric acid the crystalline molecule may be, and it seems to me 

 probably is, the chemical molecule, because if a crystal of tartaric 

 acid is dissolved and recrystallised it always remains dextro-chiral. 

 In a crystal of chlorate of soda, as has been pointed out to me by Sir 

 George Stokes, the crystalline molecule probably consists of a group 

 of two or more of the chemical molecules constituting chlorate of 

 soda, because, as found by Marbach,* crystals of the substance are 

 some of them dextro-chiral and some of them levo-chiral j and if a 

 crystal of either chirality is dissolved the solution shows no chirality 

 in its action on polarised light ; but if it is recrystallised the crystals 

 are found to be some of them dextro-chiral and some of them levo- 

 chiral, as shown both by their crystalline forms and by their action on 

 polarised light. It is possible, however, that even in chlorate of soda 

 the crystalline molecule may be the chemical molecule, because it 

 may be that the chemical molecule in solution has its atoms relatively 

 mobile enough not to remain persistently in any dextro-chiral or levo- 

 chiral grouping, and that each individual chemical molecule settles 



* 'Pogg. Ann.,' vol. 91, pp. 482-487 (1854) ; or ' Ann. de Chimie,' vol. 43 (55), 

 pp. 252255. 



