[ 4U ] 



XLIII. On the Elasticity of a Crystal according to 

 Boscovich. By Lord Kelvin, P.R.S* 



§ 1. A CRYSTAL in nature is essentially a homogeneous 

 -£j- 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 decomposition — 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 recrystallized 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 Marbachf, 

 crystals of the substance are some of them dextro-chiral and 

 some of them levo-chiral ; and if a crystal of either chirality 

 is dissolved the solution shows no chirality in its action on 

 polarized light : but if it is recrystallized 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 polarized 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 

 into either a dextro-chiral or levo-chiral configuration in the 

 act of forming a crystal. 



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

 configuration in every crystal which shows chirality in its 

 crystalline form or which produces right- or left-handed 

 rotation of the plane of polarization of light passing through 

 it. The magnetic rotation has neither right-handed nor left- 

 handed quality (that is to say, no chirality). This was per- 

 fectly 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 polarized 

 light classed together in a manner against which FaradaWs 



* Communicated by the Author ; having been read before the Royal 

 Society, June 15, 1893. 



+ Pogg. Ann. vol. xci. pp. 482-487 (1854) 

 vol. xliiir (lv.) pp. 252-255. 



