AXXTVERSARY ADDRESS OP THE PRESIDENT. 



63 



by a comparison between the immature results of the Mineralogist 

 in each of these departments and those more perfect ones which 

 have been attained by the Botanist and Zoologist. 



The Morphology of minerals was for a long time studied to the 

 exclusion of all other branches of the science ; for the problems 

 connected with form and structure were those which naturally first 

 attracted the students of the " inorganic " world. 



Few generalizations of science are so beautiful, and at the same 

 time so suggestive, as those which have been arrived at by a discus- 

 sion of the accurate measurements of crystal-angles. The constancy, 

 within certain narrow limits, of corresponding angles, amid the 

 almost infinite diversity of form assumed by crystals of the same 

 mineral, is not less striking than the simplicity of the mathematical 

 laws by which all these varied forms can be shown to be related to 

 one another. 



The actual forms assumed by crystals are often seen to be the 

 result of a struggle between opposing tendencies in the molecules to 

 build up diverse forms. In the growth of a quartz-crystal, for 

 example, the tendency towards the termination of the crystal, by 

 the formation of two rhombohedra, is being continually overcome 

 by the opposing tendency towards growth in the direction of the 

 prism-faces ; nevertheless the marks of this struggle are manifested 

 in the well-known striae on the prism-faces, each of which indicates 

 the temporary ascendancy of the rhombohedral over the prismatic 

 bias. The tendencies towards the formation of the two rhombo- 

 hedra are in turn seldom equal or able equally to assert themselves, 

 and from this cause their unequal development results. In addition 

 to these tendencies, others towards the formation of tetartohedral 

 or other faces may come into play, and the almost infinite variety 

 of forms which may thus be produced is well known to every 

 Mineralogist. 



But the study of the Morphology of minerals, which cannot be 

 carried beyond a certain point by the aid of the goniometer, is 

 capable of being pushed infinitely further when we investigate the 

 internal structure of their crystals, as illustrated by their optical and 

 other physical properties. Not only do we find the minutest details 

 of their external form to be correlated with peculiarities of mole- 

 cular structure, as revealed by their action on a beam of polarized 

 light, but delicate differences in internal organization which the 

 goniometer is- powerless to detect become clearly manifested under 



