868 Defcriplion of the Jrragonke. 



Hauy having endeavoured to divide fome of thefe cryftals mechanically, obferved that 

 they had natural planes of jundlion fuuated parallel to their prifms which indicated a ftrik- 

 ing difference between their ftru£lure and that of carbonated lime. He has alfo remarked, 

 that there is a difference of feveral degrees in the mutual incidences of the faces of the prifm, 

 and that this inequality varies indifferent cryftals. This variation of the angle would be inex- 

 plicable on the hypothefis of a fimple cryftal where there ftiould be unity of ftrudlure; but 

 the afperities diverging from the center towards the faces, and the flriae which render the 

 furface unequal, prove that they are formed of feveral cryftals grouped together in the fol- 

 lowing manner. 



The cryftal which may be confidered as the element of the group is a cuneiform ofta- 

 hedron. The four trapeziums, which form the four large faces, incline 1 16° on one fide, 

 and 64° on the other. The incidence of the triangular faces upon each other, at the place 

 where they re-unite in a common edge perpendicular to the axis, is about 70°. Thefe 

 oftaedrons are divided by feftions, parallel to their larger faces. Confidering thefe oftaedrons 

 as quadrangular prifms with dihedral fummits ; and fuppofing that their planes incline to 

 each other in an angle of 120°, it is evident that three of thefe prifms applied together will 

 form a regular hexaedral prifm whofe bafe, inftead of being plane, will prefent three faliant 

 edges which will unite at the axis of the prifm. 



But fuppofing the faces of the prifm to incline in angles of 1 16° and 64°, as is the cafe 

 in nature, then three of the prifms cannot exadly fill a fpace ; but two of them, for exam- 

 ple, A and C, fig. 3. PI. XV. will leave a concave fpace between them of 12 degrees. 



To fill this fpace the cryftallizing procefs employs a fourth prifm D, which appears to 

 penetrate in part the prifm C, fo that in the folid which refults from this re-union the two 

 faces which cut each other in the centre make an angle of 128° 5 but as real penetration is 

 impoffible the fame thing probably happens here as in thofe groups of cryftals which croffed 

 each other, and which at the place where they feem to enter each other, have a plane of 

 junflion fituated parallel to one of the faces which would be produced by virtue of a law of 

 diminution. 



The cryftals whofe bafe is covered with cuneiform projedions are nothing elfe but much 

 " more numerous groups of quadrangular prifms, whofe refpedive arrangement is fubjeded 

 to the fame conditions. 



With refpeft to thofe cryftals which have four angles of 116°, and two of 128°, the or- 

 der of their ftru£ture will be underftood by the bare infpeftion of fig. 4. In this cafe the 

 . affemblage is compofed of four prifms, A, B, C, D, which fuppofing no modification of 

 form, would leave in the interior of the prifm an empty fpace, indicated by the rhomb, h, z, 

 «, r, but this is filled by an extenfion which each prifm receives always in confequence of a 

 law of diminution ; fo that the prifm A increafes in a quantity reprefented by the rectangu- 

 lar triangle, h, 0, z, &c. 



In the cryftals whofe bafes are fmooth a diminution takes place by a fingle row on the 

 terminating edge of the component prifms. 



The 



