306 On the douhle Jlefra&ion of Rod Cryjlal. 



expofed to the fire thus produced. This, however, may have 

 been caiifed by the iron contained in the carburet ; for the 

 fufed earths and plumbago generally adhered to each other. 



There is a peculiar fpecies of native coal found on the 

 banks of the Lehigh in this ftate, which it is extremely dif- 

 ficult to ignite ; but when expofed to a high degree of heat, 

 and a copious blart of air, it burns, yielding an intcnfe heat 

 without either fmoke or flame, and leaving little refidue. By 

 expofure to the gafeous flame on this coal, both magnefia and 

 lime exhibited Itrong fvmptoms of fufion. The former af- 

 fumed a glazed and fomewhat globular appearance. The 

 latter became converted into a brownidi femivitreous mafs. 



The heat of the gafeous flame is very much dependent on 

 the proportional quantities of the gafcs emitted. On this 

 account, the perforations in the keys of the cocks N, n, 

 fig. ifl^, fliould be narrow and oblong, to admit of a more 

 gradual increafe or diminution in the quantity of gas emitted. 



I have now concluded my communications on the fubjedl 

 of this paper, and fliall be happy if they have heen found 

 worthy of the time and attentiun beRowed on them by the 

 fifcietv. 



XLVIII. On the dijuhle Befra&lon of Rock Cryfal, and an- 

 other dhplric Property of that Mineral Suhjlance. By C. P. 

 TouELH DE Nakci, attached to the Council of Mines*, 



X-/OUBLE refraftion, that Angular property of rock cryf- 

 tal t, and of feveral other minerals, has long engaged the at- 

 tention of mineralogills and philofophers, though they em- 

 ployed it in no other manner than as a diftinguifliing cha- 

 radicr. C. Haiiy fays J : " It would be difficult to find a 

 more ftriking chara6ler than that deduced from double re- 

 fraftion, fince it depends on the eifence of the minerals in 

 which it exills." 



C. Rochon is the fird philofopher who employed this pro- 

 perty of rock cryftal to meafure fmall angles; and he read ou 

 the 26th of January and 9th of April 1777, to the Academy 

 of Sciences, two memoirs on the application he made of it, 

 and the exaft refults he obtained. This dilcovery muft be 

 of the greateft utility, if means flionld be found to conftruft 

 the inftrument he invented at fo moderate a price as to bring 



* 

 "" From the Joional eks Minn, No. 6f). 



\ Limpid hv:ilin qviartz, Hauy Traiid de Mineralcgie , vol. ii. p. 4.27. 

 \ Vol. i; of his Trcatife, p 254. 



ft 



