332 HISTORY OF CHEMISTRY. [LECTURE XV. 



and by Maxwell, 202 and on the transpiration of vapours by 

 Lothar Meyer. 203 There still remain to be mentioned, the funda- 

 mental investigations of Biot upon the rotation of the plane of 

 polarisation, 204 and the researches connected with them, by 

 Landolt 205 and others ; and also the experiments of Perkin on 

 the electro-magnetic rotation of the plane of polarisation. 200 



Finally, I must refer in a few words to relations which 

 have been discovered between crystalline form and chemical 

 composition, a consequence of which may be a considerable 

 expansion of the idea of isomorphism. The credit of having 

 discovered these relations belongs to Groth ; 20T and his views 

 have been extensively confirmed by means of the numerous 

 researches by himself and his pupils. Groth follows out the 

 changes of the axial ratios which take place upon the entrance 

 of substituting groups, and in this way arrives at definite laws. 

 He gave the name morphotropy to the phenomena, and caused 

 experiments to be made in order to determine the morpho- 

 tropic influence of definite substitutions. The morphotropic 

 effect of chlorine, bromine, and iodine, for example, proved to 

 be analogous to that of hydrogen ; and hence these elements 

 have been designated isomorphotropic. 208 It was then an- 

 nounced by Hintze 209 that isomorphism might be regarded as 

 a special case of morphotropy ; a point to which Groth had, 

 however, already directed attention. 



302 Phil. Trans. 1866, 249. '- 203 Wieclem. Ann. 7, 497 ; 13, i. ' M Ann 

 Chim. [3] 59, 206. %205 Das optische Drehungsvermogen organischer Sub- 

 stanzen, 1879. ' m J. pr. Chem. [2] 31, 481 ; 32, 523. i207 Pogg. Ann. 

 141, 31 ; Berichte. 3, 449 ; compare, however, Laurent, Comptes Rendus. 

 J 5> 35l 2 357 ; Methode de chimie. 156; E., 129. -208 Hintze, Pogg. 

 Ann. Erganzungsband 6, 195. 209 Habilitationsschrift. Bonn 1884. 



