AXX1VEESAEY ADDRESS OE THE PRESIDENT. 



65 



growths and enclosures ; and the study of these, as shown by Sorby 

 Vogelsang, Eenard, and Xoel Hartley, may serve to throw new and 

 important light upon the conditions under which the crystals were 

 originally developed. Cavities containing carbonic acid and other 

 liquids, with bubbles in constant and, seemingly, spontaneous move- 

 ment, serve to awaken the interest of the naturalist not less power- 

 fully than the mysterious creeping of protoplasm in the hair of a 

 nettle, or the dance of blood-corpuscles in tbe foot of a frog ! 



Others among these histological peculiarities of crystals must be 

 regarded as having a pathological significance ; they are abnormal 

 developments resulting from unfavourable conditions to which the 

 crystals may have been subjected during their growth, or in the 

 course of their long and checkered existence. 



The variability exhibited in crystals of the same mineral is some- 

 times very startling. In addition to the varieties due to the 

 combinations of many different forms, or to the excessive development 

 of certain faces at the expense of others, we have the complicated 

 and diversified structures built up by twinning according to different 

 laws. Again, by oscillatory tendencies in the same crystal towards 

 the assumption of different forms, or by the existence of causes 

 calculated to interfere with the free action of the crystallizing 

 forces, we may obtain varieties with curiously striated or curved faces. 

 Xot unfrequently large quantities of extraneous materials, solid, 

 liquid, or gaseous, may be caught up in the crystal during its 

 growth, and these foreign substances may be so far affected by the 

 polar forces operating around them as to be made to assume definite 

 and symmetrical positions within the crystal. 



Even in the case of minerals of identical chemical composition and 

 similar crystalline form, marked variations in physical properties 

 may result from differences in the conditions under which they have 

 originated. In lustre, density, and other characters, Adularia differs 

 from Sanidine, and Elaeolite from Nepheline. Dr. Arthur. Becker 

 has shown that Quartz exhibits marked variations in its specific 

 gravity, according to the particular conditions under which it has 

 been formed. 



There is one kind of morphological variability in minerals which 

 has during recent years attracted a great amount of attention, and 

 excited much discussion among Mineralogists. Soon after his 

 memorable discovery of the relations between the crystalline forms of 

 minerals and their optical properties, Brewster detected certain appa- 

 rent exceptions to his important generalization : and since his day 



vol. xliii. /' 



