192 REFLECTIVE GONIOMETER. 



manner represented at fig. 6, plate IV, has an iron recep- 

 tacle, A, ground into one end of it, and furnished with a 

 ground stopper. This receptacle contains from two to three 

 ounces of potash (pure potash) that has been in fusion. 

 Verj^ clean iron turnings are placed in the middle of the 

 barrel D, which is heated to whiteness ; and the potash is 

 kept cool till this temperature is attained. The potash is 

 then slowly fused, and it passes through a hole in the re- 

 ceptacle upon the turnings. The heat is kept up till gaseous 

 matter ceases to come over. The extremity of the lube B 

 is kept cool, and in this the potassium collects. Common 

 air is prevented from ei^ring by a glass tube, C, which 

 supports the weight of a column of mercury. 



VIII. 



Description of a rcjlective Goniometer. By William Hyde 

 WoLLAbTON, M.D. Sec. R.S*. 



J[/ ROM the advances that have been made of late years in 

 crystallography, a very lar2:e proportion of mineral substances 

 phy muf li ad- ^'^Y "^w be recognised, it we can ascertain the angular di- 

 vanced. mansions of their external forms, or the relative position of 



those surfaces, that are exposed by fracture. But thouo^h 

 the modifications of tetrahedrons, of cubes, and of those 

 other regular solios, to which the adventitious aid of geo- 

 metry could be correctly applied, have been determined 

 with the utmost precision, yet ii has been often a subject of 

 regret, that our instruments for measuring the angles of 



crystals are not possessed of equal accuracy, and that in 

 Sut a good in- ' , . , 



strumentfor apply»"g the goniometer to small crystals, where the radius 



irieisunng the jjj contact with the surface is necessarily very short, the 



talsstUi want- measures, even when taken with a steady hand, will often 



«"'?• deviate too much from the truth to aid us in determining 



the species to which a substance belongs. 

 ^ A means of remedying this defect has lately occurred to 



mCi by which in most cases the inclination of surfaces may 



Thi-^ rlefc'Ct 



**^^»*'"^'*- * Philos. Trans, for 18G9, p. 253. 



fee 



