On the Hemihedrism of Cuprite. 127 



Perhaps the most salient fact concerning this group is that 

 the methyl compound is intermediate in these of its physical 

 properties between the ethyl and the amyl members. So 

 interesting did this relationship appear, that we have deter- 

 mined the specific gravities of the three salts. Turpentine 

 was used as the liquid displaced. The dry salt having been 

 weighed in the specific-gravity bottle, a little turpentine was 

 added and the air exhausted. The bottle was subsequently 

 filled with turpentine in the usual way. The air-temperature 

 was 19 0, 6, and the specific gravity of the turpentine compared 

 with water at the same temperature was 0*86838. It was 

 found that 



fsp. grav. of sulphomethylate of potassium = 2 097, 



at 19°*6< „ „ sulphcethylate of potassium =1*843, 



[ „ „ sulphoamylate of potassium =1*144. 



Accordingly the densities of the salts are in the inverse 

 order of their molecular weights, and the ethyl compound is 

 in this respect in its usual place between the amyl and methyl 

 relatives. 



Physical Laboratory, Science Schools, 

 South Ken sing-ton. 



XV. Hemihedrism of Cuprite. By H. A. MlERS, M.A , 

 Assistant in the Mineral Department, British Museum*. 



[Plate VI.] 



A LOOSE crystal of cuprite (about 4 millim. across], 

 found among a large number of mineral specimens 

 which had been collected by the late Mr. Richard Tailing, 

 and which were offered to the British Museum for purchase, 

 was noticed by Mr. Thos. Davies as hemihedral in form. 



Subsequently about twelve specimens were found, all from 

 Wheal Phoenix, Cornwall, belonging partly to the same col- 

 lection and partly to the collection in the Museum, which are 

 characterized by the same peculiarity. This hemihedral de- 

 velopment I have been requested by Mr. Fletcher to describe. 



It proves to be a mode of hemihedrism of great interest 

 to the crystallographer as one which for more than sixty 

 years has been regarded as theoretically possible, but which 

 has not been hitherto observed on any mineral, and has only 

 recently been proved to exist on artificial sal-ammoniac. 



The usual form of the crystals is shown in fig. 4, where a 

 are the cube, o the octahedron, and x the new hemihedral 



* Communicated by the Author. 



