MINERALOGY. 629 



the species, nearly the same angles. The advantage to be obtained by 

 such a change would be the simplification of the laws of derivation in 

 the derivative forms : and therefore we have to ask, whether the indices 

 of derivation are smaller numbers in this way or with the hitherto 

 accepted fundamental angles. It appears to me, from the examples 

 given, that the advantage of simplicity in the indices is on the side of 

 the old system : but whether this be so or not, it was a great benefit 

 to crystallography to have the two methods compared. Mr. Brooke's 

 Essay is a Memoir presented to the Royal Society in 1856. 



2. Optical Properties of Minerals. 



The Handbuch der Opt'ik, von F. W. G. Radicke, Berlin, 1839, con- 

 tains a chapter on the optical properties of crystals. The author's 

 chief authority is Sir D. Brewster, as might be expected. 



M. Haidinger has devoted much attention to experiments on the 

 pleochroism of minerals. He has invented an instrument which makes 

 the dichroism of minerals more evident by exhibiting the two colors 

 side by side. 



The pleochroism of minerals, and especially the remarkable clouds 

 that in- the cases of lolite, Andalusite, Augite, Epidote, and Axinite, 

 border the positions of either optical axis, have been most successfully 

 imitated by M. de Senarrnont by means of artificial crystallizations. 

 (Ann. de Chim. 3 Ser. xli. p. 319.) 



M. Pasteur has found that Racemic Acid consists of two different 

 acids, having the same density and composition. The salts of these 

 acids, with bases of Ammonia and of Potassa, are hemihedral, the 

 hemihedral faces which occur in the one beino- wantino- in the other. 



O O 



The acids of these different crystals have circular polarization of oppo- 

 site kinds. (Ami. de Chim. 3 Ser. xxviii. 56, 99.) This discovery 

 was marked by the assignation of the Rumford Medal to M. Pasteur 

 in 1856. 



M. Marbach has discovered that crystals of chlorate of soda, which 

 apparently belongs to the cubic or tessular system, exhibit hemihedral 

 faces of a peculiar character ; and that the crystals have circular polar- 

 ization of opposite kinds in accordance with the differences of the 

 plagihedral faces. (Poggendorfs Annalen, xci. 482.) 



M. Seybolt of Vienna has found a means of detecting plagihedral 

 faces in quartz crystals which do not reveal them externally. (Akad. 

 d. Wissenschaft zu Wien, B. xv. s. 59.) 



