254 SCIENCE PROGRESS. 



a crystal of calcite by mere pressure into two crystals united 

 by a twin-plane ; any one can repeat the experiment, as 

 suggested by Baumhauer, by merely pressing a knife-blade 

 into the edge of a calcite-rhomb, and the marvellous 

 manner in which the part of the crystal which is pressed 

 changes as though by magic to a new crystal must delight 

 any who see the experiment performed ; from this simple 

 experiment have resulted modern conceptions of " gliding- 

 planes " and "secondary twinning" produced in crystals by 

 changes of temperature and pressure ; this secondary twin- 

 ning has now been produced in many substances and has 

 been experimentally studied by Miigge ; it has been 

 observed in many minerals and is not without geological 

 significance as indicating the former existence of pressure ; 

 it has moreover an important bearing upon theories of 

 crystal structure,' and has provoked the recent suggestive 

 work of Lord Kelvin upon the "Molecular Tactics of Iceland 

 Spar " (10). 



Equally simple and striking is the experiment of Mallard 

 upon boracite, a mineral which though apparently cubic 

 exhibits double refraction ; he found that when heated to 265° 

 the crystals become isotropic ; it is impossible to see this 

 experiment without being led to inquire into the molecular 

 structure which can explain so remarkable a phenomenon, 

 and it has given rise to a flood of research. 



Or take again that other experiment of Reusch, in which 

 a number of mica plates are piled one above the other in 

 the fashion of a spiral staircase ; such a combination is 

 found to rotate the plane of polarisation of a plane polarised 

 ray of light just as it is rotated by a crystal of quartz ; the 

 experiment affords a striking justification of, if it did not 

 actually prompt, the views of Sohncke and Mallard regard- 

 ing the structure of quartz and other optically active 

 crystals, according to which these crystals are composed of 

 a spiral arrangement of particles (11). 



Again it has generally been supposed that absolute 

 repose is essential to the growth of well-developed crystals ; 

 in reality, however, direct experiments of Wulff have shown 

 that under some conditions continual movement is an 



