184 



degree of mechanical stability of the total molecular arrangement . 



7. It was such facts as these, that have led Mallard since 

 1876 to his most suggestive explanation of a number of optical 

 phenomena exhibited by numerous crystalline substances, namely 

 the occurrence of the above mentioned optical anomalies and the 

 rotatory power in uniaxial crystals belonging to the trigonal, tetra- 

 gonal, and hexagonal systems. 



Optical anomalies have been observed in a gradually increasing 

 number of crystals ever since Brewster *) in the beginning of the 

 nineteenth century first discovered and studied them. More especially 

 the symmetry of the optical behaviour of such crystals appeared 

 to be appreciably lower than that of their external forms; or, what 

 is another view of the same fact : their geometrical form is evidently 

 of higher symmetry than that of their internal molecular structure. 



Thus, many crystals of the cubic system are birefringent, and, 

 in striking contrast to what might be expected, they act powerfully 

 upon transmitted polarised light; tetragonal and hexagonal crystals 

 are notoriously biaxial, and show optical phenomena analogous to 

 those to be expected in rhombic, monoclinic, or triclinic crystals; etc. 



The apparently cubic minerals: boracite, leucite, perowskite, fluor- 

 spar, diamond, garnet, analcite, etc., are in most cases distinctly bire- 

 fringent, and the same is true for many laboratory-products, such 

 as alums, the nitrates of barium, strontium, and lead, Schlippe's 

 salt, sodium-chlorate, sodium-br ornate, etc., all crystallising in one 

 of the classes of the cubic system. The apparently tetragonal crystals 

 of potassium- f err ocyanide, of strychnine- sulphate etc., and of minerals 

 like idocrase, apophyllite, etc., are beyond all doubt optically biaxial. 

 The same is true for a great number of substances which, with respect 

 to their crystal-forms, must belong to the trigonal or hexagonal 

 systems, as for instance : quartz, turmaline, chabazite, sodium-periodate, 

 beryll, apatite, the dithionates of potassium, rubidium, caesium, 

 calcium, strontium, and lead, and many other chemical compounds. 



The disparity between the optical character of such crystals and 

 their geometrical appearance is, therefore, an indisputable fact, and 

 even one of frequent occurrence ; and a very great number of highly 

 interesting investigations have been made with the purpose of eluci- 



!) D. Brewster, Phil. Trans. London, /, 187, (1814); Trans. Royal Soc. 

 Edinb., 8, /, 155, (1817); Phil. Trans. London, /, 199, (1818); Trans. R. Soc. 

 Edinb., 9, /, 139, (1821); II, 317, (1823); 10, 187, (1826); Edinburgh Phil. Journ., 

 1, 1, (1819); 3, 98, (1820); 5, 217, 218, (1821); Phil. Mag., 7, 245, (1835); etc. 



