50 On the Homceomorphism of Mineral Species 



tially homoeomorphous, as well as similar in chemical formulas. 

 In both of these cases, one of the species contains fluorine, and 

 this is evidently the occasion of the wide divergence. Yet in 

 one instance the fluorine species (chondrodite) belongs to section 

 I., and in the other (topaz) to section II. 



The table affords examples, also, of the principle stated in a 

 preceding page, that homoeomorphous species, while identical in 

 the particular axis which is the vertical, may vary by a simple 

 ratio (1 : 2 or 2 : 3) in the axes, and that they are to be recog- 

 nised as species that belong to a specific system of ratios, rather 

 than to definite and identical dimensions. 



Andalusite, Staurotide, and Topaz, have this relation. The 

 forms of these species may be referred to a similar type ; yet 

 we cannot affirm that the axes have the near identity presented 

 in the table, rather than a multiple ratio of 1 : 2 in some of the 

 axes ; we only know that they pertain to a common series. 



Staurotide alone offers a choice between three uncertainties. 

 The occurring form is a prism of 129° 20' ; and this is usually 

 taken as the unit vertical prism. A prism with the longer 

 lateral axis half as long, has the angle 93° 8', and this approaches 

 the prism of Andalusite ; and as the frequency of occurrence 

 of a plane is no sure proof that the plane is necessarily of the 

 fundamental series, we may with some reason assume the prism 

 of 93° 8' for the fundamental one. But Staurotide forms twins 

 in two directions, or parallel to two planes, and neither of these 

 planes, referred to the above fundamental forms, has a simple 

 ratio or expression, and this, notwithstanding the general fact 

 that the faces of composition are of the highest value in ascer- 

 taining the directions of axial sections ; moreover, one of the 

 planes has the unusual symbol f § if referred to the prism of 

 129- 20', and \ | if referred to that of 93° 8'. Now, if instead of 

 halving the longer lateral axis, we take two thirds for the new 

 axis c, then the expression is of the simplest kind in every re- 

 spect. The following are the angles and symbols of the planes 

 according to these three methods : — 



