TERMINOLOGY. . 159* 



the aspect of a combination of Pr. P. (Pr + oc) 3 . Pr + co of 

 the prismatic system, much resembling Vol. II. Fig. 16. 

 Also in this case the means quoted above will secure us from 

 errors which a little practice very soon teaches to avoid in 

 imperfectly formed varieties of crystallisations. 



The curvature of faces, if occurring in simple forms, 

 in general affects all the faces at once. Thus it is in the 

 hexahedrons of octahedral Fluor-haloide ; in the dodeca- 

 hedrons, icositetrahedrons, and tetracontaoctahedrons of 

 octahedral Diamond ; in the rhombohedrons of the two 

 species of Parachrose-baryte, &c. Similar imperfections 

 produce the lenticular forms, particularly the saddle-shaped 

 lens of the above mentioned species ; which is more correctly 

 represented in Fig. 79- than in any of the drawings and 

 models hitherto published, in which, for the greater part, it 

 is given with four corners, instead of six. 



In combinations, the curvature always takes place upon 

 homologous faces, while the rest are not affected by this 

 deformity. Examples may be found in the species of pris- 

 matoidal Gypsum-haloide, of paratomous Augite-spar, of 

 octahedral Diamond, &c. 



Curved edges are produced by the intersection of curved 

 faces ; rounded edges, too, arise evidently from the curva- 

 ture of the adjoining faces. 



From the preceding considerations of the irregularities 

 of crystals, it is plain that it is necessary to observe the 

 greatest precautions in ascertaining the measures of their 

 angles, if we wish to obtain useful and correct results. 

 The inaccuracies of so many of these measurements, are not 

 always errors arising from the imperfection of the instru- 

 ment, or from the operation ; but very often they are the 

 consequences of the imperfection of the crystals them- 

 selves. Small crystals are commonly less subject to these 

 irregularities than large ones ; and to this, in particular, 

 the great advantage must be ascribed, which the lleflective 

 Goniometer possesses over the common one, because, in 

 applying the former, we may make use of crystals which 

 are smaller, and therefore in general more perfectly formed, 



