AXXIVERSART ADDRESS OF IHE PRESIDEXI. 



7 1 



parallel to the gliding-plane, are actually made to rotate through an 

 angle of 180°. 



At one time these M gliding-planes " were regarded as being 

 peculiar to a few minerals, such as calcite and rock-salt ■ but the 

 investigations of Frankenheim, Baunihauer, Foerstner, and especially 

 of Miigge, have shown that they exist in crystals belonging to every 

 group in the mineral kingdom, including all those minerals which 

 occur as common rock-forming constituents, such as the felspars and 

 pyroxenes. 



As is the case with the cleavage-planes, so with the gliding- 

 planes, there may exist one, two, or three in the same crystal. 

 One of these is usually a principal gliding-plane — the slipping 

 movement with its accompanying twin-lamellae being produced 

 parallel to it with the greatest facility — while the others are sub- 

 ordinate ones. 



Strange to say, however, the particular gliding-plane along which 

 a crystal yields appears to be determined, not only by the direction 

 in which the force is applied, but to some extent also by the nature 

 of that force, whether percussive, or a sustained pressure, or a 

 violent stress ; in some cases where the application of external 

 force fails to produce the gliding movement with its accompanying 

 lamellar twinning, it may be induced by the strains which result 

 from unequal expansion and contraction during the hearing and 

 cooling Qf a crystal. Some mineralogists have, indeed, proposed 

 to apply distinctive names to the results which follow from the 

 application of different kinds of force — whether a blovr (Schlag- 

 figuren), pressure (Reissflachen), or the effect of heating and cooling 

 ( Contractions risse). 



The gliding-planes of crystals are quite distinct from the cleavage- 

 planes, though some very curious and interesting relations have, in 

 certain cases, been shown to exist between them. That the arti- 

 ficial formation of twin-lamella?, like the production of cleavage, 

 is rendered possible by complicated molecular structures, it is 

 scarcely necessary to point out. The application of external force 

 to such crystals is like the putting of a spark to a train of gunpowder : 

 the molecules lying in parallel bands are in unstable equilibrium, 

 ready, so soon as set in motion, to roll through an angle of 180°. 



There is still a third and even more subtle set of structure-planes 

 in crystals to which I must now allude, those, namely, for which 

 the name of solution-planes has been proposed. 



It was long ago shown by Daniell that when crystals are exposed 



