March 3, J 887] 



NA rURE 



4'5 



light-waves through their substance leads to permanent 

 molecular rearrangements which are evidenced by marked 

 changes in colour, translucency, and other properties. 



Many minerals have their atoms so arranged that the 

 action of external forces causes them to fall readily into 

 new combinations. In this way there are brought about 

 such paramorphic changes as that of aragonitc into 

 calcite, and au.^ite into hornblende. Excessively slight 

 manifestations of force are sometimes sufficient to induce 

 such paramorphic changes. 



But the most significant fact of ali is that every crystal 

 possesses certain peculiarities of molecular structure, and 

 as the result of this internal "organisation,'' it responds 

 in a definite manner to the action of various external 

 forces, undergoing in this way well-marked series of 

 physical and chemical changes without losing its identity. 

 As the final result of such successive changes, however, 

 the bonds which hold the " organised " structures together 

 are gradually weakened, and at last break down altogether. 

 In this way the separate existence of the mineral comes 

 to an end ; but the materials of which it was composed, 

 resolving themselves into new compounds, may go to 

 build up the substance of other " organised " structures. 

 Need I point out that in all these respects minerals behave 

 exactly like plants and animals? 



But in the case of plants and animals changes such as 

 these, which are the direct outcome of externnl forces 

 acting on a special organisation, are caWeA physiologiial, 

 and I know of no valid reason why the same term should 

 not be employed in the case of minerals. It is true that 

 the accomplishment of the cycles of change in minerals 

 often requires periods of time of enormous duration, and 

 that during incalculable intervals they may appear to be 

 wholly suspended ; but in these respects the " life " of a 

 mineral differs from that of a plant in just the same manner 

 as the latter does from the life of an animal. 



I must ask your attention for a few moments to these 

 peculiarities of internal organisation in minerals, and to 

 the way in which the various physical and chemical forces 

 act and react upon them in consequence of their special 

 organisation. 



Recent researches have shown that every crystal pos- 

 sesses a number of planes, all of which are related to its 

 peculiar symmetry, along which the several physical forces 

 operate in a marked manner to produce ch inges in the 

 physical and chemical properties of the crystal. These 

 planes have been called the "structure-planes" of the 

 crystal. 



By far the most obvious of these structure-planes of 

 crystals are those of cleavage. When crystals are subjected 

 to the action of mechanical force they break up along one, 

 two, or three definite planes, with varying degrees of ease. 

 In some cases when this separation cannot be readily 

 effected by percussion or pressure, it may be brought 

 about by the unequal expansion and contraction in a 

 crystal resulting from alternate heating and cooling. We 

 cannot arrive at the limit of this liability of a crystal to 

 separate along its cleavage-planes ; if we powderacalcite- 

 crystal and examine the fine dust under a microscope, 

 each minute grain will be seen to have the form of a 

 cleavage-rhomb of the material. 



Now the exquisite molecular structure of a crystal, of 

 which this wonderful property of cleavage is the outcome, 

 is borne witness to, not only by the perfection of the 

 cleavage-surfaces— presenting, as they do, a lustre which 

 no artificial polish can imitate— but by the fact that each 

 particular set of cleavage-surfaces presents definite charac- 

 teristics, analogous to those seen in the actual faces of 

 crystals. Each exhibits striking peculiarities in its mode 

 of reflecting light ; each yields in varying degrees to a 

 hard point drawn across it in different directions ; and 

 each, when treated with appropriate solvents, is attacked 

 in a characteristic fashion, giving rise to the geometrical 

 forms known as the etching-figures. Wonderful as these 



cleavage-surfaces are, however, it must be remembered 

 that the power of cleavage is one that, under ordinary 

 circumstances, remains altogether late/il in crystals. 



Cleavage-planes, however, are not the only latent 

 structure-planes in crystals. Long ago it was shown by 

 Brewster, Reusch, and Bfaff, that when minerals are sub- 

 jected to pressure in certain directions, their molecules 

 appear to glide over one another along certain definite 

 planes within the crystal ; and, if we examine optically a 

 crystal which has been treated in this manner, it is actually 

 found to exhibit a series of twin-lamells arranged parallel 

 to the so-called "gliding-planes." It thus appears that in 

 the movements set up within a crystal by the application 

 of force from without, certain of the molecules of which 

 the crystal is built up, lying in bands parallel to the- 

 gliding-plane, are actually made to rotate through an 

 angle of i %6\ 



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

 being peculiar to a few minerals, such as calcite and rock- 

 salt ; but the investigations of Frankenheini, Baumhauer, 

 Foerstner, and especially of Mugge, 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 feldspars 

 and pyroxenes. 



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

 gliding-planes, there may ev;ist one, two, or three in the 

 same crystal. One of these is usually a principal gliding- 

 plane — the slipping movement with its accompanying twin- 

 lamellK being produced parallel to it with the greatest 

 facility — while the others are subordinate 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 per- 

 cussive, 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 accompinying 

 lamellar twinning, it may be induced by the strains which 

 result from unequal expansion and contraction during the 

 heating and cooling of 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 blow {Sc/i!agjigure/i), pressure 

 [Rcissfiac/wn), or the effect of heating and cooling 

 ( Coutractionrisse). 



The gliding-planes of crystals are quite distinct from 

 the cleavage-planes, though some very curious and inter- 

 esting relations have in certain cases been shown to exist 

 between them. That the artificial formation of twin- 

 lamelte, 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 i8q°. 



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

 ture-planes in crystals to which 1 must now allude, those, 

 namely, for which the name of sotiition-plivics has been 

 proposed. 



1 1 was long ago shown by Daniell that when crystals 

 are exposed to the action of solvents they are attacked in 

 such a manner as to give rise to peculiar geometrical 

 forms. The subject has been followed up by Baumhauer, 

 Leydolt, Becke, and others, who have shown what a 

 wonderful variety of " etching-figures" may be produced 

 by operating upon the various faces and cleavage-surfaces 

 of different crystals. 



Ouite recently, however, it has been showi\ by Von 

 Ebner, as the result of his studies of calcite and aragon- 

 ite, that all the complicated phenomena of the etched 

 figures arise from the existence of planes along which 

 solvent or chemical action takes place mo^t readily within 



