Feb. 24, 1S87] 



NA TURE 



395 



organisation which the goniometer is powerless to detect, 

 become clearly manifested imdcr the searching tests of 

 optical analysis. For the mineralogist, indeed, the polari- 

 scope with its accessories has supplemented the gonio- 

 meter, in the same way as the spectroscope has the 

 balance of the chemist. 



What has been stated concerning the optical characters 

 of minerals is equally true of their other physical pro- 

 perties : for the researches of recent years have shown all 

 these properties to be intimately related to the symmetry 

 of the crystal in which they are displayed. In every crys- 

 tal, the faces of each group bearing the same relations to 

 its axes exhibit characteristic peculiarities in their lustre, 

 in their hardness, and in the manner in which they are 

 acted upon by solvents ; and these serve to distinguish 

 such groups of faces from others in the same crystal 

 having different relations to its axes. The elasticity of 

 crystals, their power of conducting heat and electricity, 

 and their phosphorescent, electric, or magnetic properties, 

 whether natural or induced, are all manifested in varying 

 degrees along certain directions which can be shown to be 

 related to the particular symmetry of the crystal. And 

 the more carefully we study both the forms and the 

 physical properties of minerals, the more are we impressed 

 by the conviction that the most intimate relations exist 

 between these characters and the chemical composition of 

 the minerals. 



The phenomenon of " plesiomorphism," as Miller pro- 

 posed to call it, that is, the slight variation in the angular 

 measurements of crystals in the same species or group, 

 when any of the constituents are replaced by vicarious or 

 isomorphous representatives, very strikingly illustrates 

 this conclusion. And the exact study of the optical 

 properties of minerals shows that the slightest variation 

 in the relative proportions of these vicarious constituents 

 makes its influence felt by changes in their colour, in 

 their pleochroism, in the nature and amount of their 

 double refraction, in the position of their optic axes, and, 

 indeed, in the whole assemblage of the properties of the 

 crystal. 



To the admirable investigations of Tschermak on the 

 feldspars, the amphiboles and pyroxenes, the micas, and 

 other groups of minerals, we are largely indebted for the 

 establishment of this conclusion ; while Doelter, Max 

 Schuster, and other mineralogists, have contributed many 

 striking observations which serve to extend and fortify it. 



The application of the microscope to the study of the 

 internal structure of minerals — their histology — has led to 

 the recognition of many beautiful and unsuspected pheno- 

 mena. Examined in this way, the seemingly homogeneous 

 masses exhibit many interesting intergrowths and in- 

 closurcs ; and the study of these, as shown by Sorby, 

 \'ogclsang, Renard, 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 movement, serve to awaken 

 the interest of the naturalist not less powerfully than the 

 mysterious creeping of protoplasm in the hair of a nettle, 

 or the dance of blood-corpuscles in the 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 unfavour- 

 able conditions to which the crystals may have been 

 subjected during their growth, or in the course of their 

 long and chequered existence. 



The variability exhibited in crystals of the same mineral 

 is sometimes very startling. In addition to the varieties 

 due to the combinations of many different farms, or to 

 the excessive development of certain phases at the expense 

 of others, we have the complicated and diversified struc- 

 tures built up by twinning according to dirterent laws. 

 Again, by oscillatory tendencies in the same crystal 

 towards the assumption of diflferent forms, or by the 



existence of causes calculated to interfere with the free 

 action of the crystallising forces, we may obtain varieties 

 with curiously curved or striated faces. Not 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 varia- 

 tions in physical properties may result from difterences in 

 the conditions under which they have originated. In 

 lustre, density, and other characters, adularia differs from 

 sanidine, and ela^olite 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 

 apparent exceptions to his important generalisation ; and 

 since his day many additions to these curious anomalies 

 in the optical behaviour of minerals have been made by 

 other observers. So greatly, indeed, hax-e these bsen multi- 

 plied in recent years, that it is doubtful whether any mineral 

 crvstallising in the cubic, the tetragonal, or the hexagonal 

 system could be cited in which the optical properties are 

 precisely what they ought to be according to theory ; and 

 similar anomalies are also found in crystals possessing 

 lower degrees of symmetry. 



The attempts \\-hich have been made by some crys- 

 tallographers to account for these optical anomalies in 

 ciystals, by assuming that they possess only a pseudo- 

 symmetry-, the result of ver)- complicated twinning, 

 ingenious as they undoubtedly are, remind one of the 

 wonderful addition of eccentrics and epicycles by which 

 astronomers so long sought to maintain the credit of the 

 Ptolemaic theory. But as, in the latter case, complexities 

 and difficulties 'alike vanished when the centre of the 

 system was shifted from the earth to the sun, so have 

 the discoveries of Klein, Rosenbusch, and others removed 

 the necessity for the painfully elaborate crystallographic 

 hypotheses to which we ha\-e referred. 



' Most mineralogists will now be prepared to admit, as 

 the result of these researches, that the perfection alike of 

 form and of optical properties which characterises a crystal 

 when first formed, is liable to slight modification, as the 

 conditions of temperature and pressure under which it 

 exists vary. In consequence of this, almost all natural 

 crystals are found, when we study them with sufficient 

 care, to exhibit slight but very striking and significant 

 differences in form and optical behaviour from what they 

 ought theoretically to possess. 



While our knowledge of the ordinary mineral varieties 

 promises to be vastly extended by the improvements 

 which have been made in the methods of optical and 

 chemical diagnosis under the microscope, there is, at 

 the s;une time, reason to hope that the relationship of 

 these numerous varieties will, by the s.ime means, be 

 made more clearly apparent. As the existence of well- 

 defined natural groups of minerals becomes more clearly 

 established, through the study of interesting though in- 

 conspicuous links, we shall obtain a basis for a much- 

 needed reform in mineral taxonomy and nomenclature. 



The more carefully we pursue our researches among 

 the diversified forms of the mineral world, the more are 

 we impressed by the conviction that each mineral, like 

 each plant or animal, possesses its own individuality. 

 Nature does not m?k^facsimi/es in the mineral, any more 

 than in the vegetable or the animal kingdoms. All the 



