550 Analyfis if the Spinel. 



derable variations. All thefe different cryftallizations are very carefully defcribed by the 

 Abbe Eftner *. 



The colour of this ftone is alfo much varied, and pafles through all the gradations of red. 

 This variety of colour has caufed lapidaries to diftinguifti it by different names, fuch as the 

 ruby, almandin, fpinel, balais, rubicel. 



t The red colour of this precious ftone is not only very fixed in the fire, but it even becomes 

 ■deeper by careful ignition of the pale varieties. According to the teftimony of Julius Sca- 

 lio^er f, the inhabitants of Zealand are very well acquainted witli the method of profiting by 

 this, which is, perhaps, the only reafon why we fo feldom .obferve in the rough ruby the fine 

 and beautiful colour of thofe which have been cut. 



With reo-ard to their colour, thofe fpinels are the moft rare which are beautifully tranf- 

 parent and colourlefs, fuch as the perfect oftahedron which Mr. Macie of London preferves 

 in his colleftion ; in the next place are thofe of a fapphire-blue, fuch as that of iVIr. F. Gre- 

 viile; and next the green, as that pofleffed by Mr. Hawkins, &c. This ftone affords a new 

 proof that the colour of precious ftones is a very inferior charafter for determining their 

 claflification. 

 •I found the fpecific gravity of the fpinel in feleil cryftals to be from 3,570 to 3,590. 



Though I have publiftied an analyfis of the fpinel feveral years agoj, there are neverthe- 

 'lefs feveral matters which were not, at that time, well known, and appeared to me to re- 

 quire a hew courfe of experiments. The event has (hewn me that, in my former work, I 

 had overlooked a fubftance not fufpeded to exift in this ftone, namely, magnefia. Among 

 Other analyfes made to determine the quantity of this new principle, I fliall here give that- 

 Which appeared to me to deferve the. moft confidence. 



A. One hundred grains of rough fpinel of Zealand, in feledl cryftals, were pounded in 

 a fteel-mortar, and then reduced to an impalpable powder by grinding them in a mortar of 

 filex with a little water. The powder being then dried and flightly ignited was found to 

 have gained nine grains by erofion from the mortar. 



B. On this powder I poured 960 grains of muriatic acid, and fet them to digeft. After the 

 acid was evaporated nearly to drynefs, I diluted the mafs with water, and poured the whole 

 on a filtre. The acid liquor was yellow. I fuperfaturated it with ammoniac, which threw 



'down brow^i •flocks of iron, that weighed one quarter of a grain after ignition. 



C. The liquor feparated from this precipitate was evaporated, faturated with muriatic acid, 

 and mixed with oxalate of potafti. A precipitate of oxalate of lime fell down, which was 

 carefully colledted, pteced in a hole in charcoal, and ignited by the blow-pipe. It afforded 

 three quarters of a grain of pure calcareous earth. When this was diffolved in the nitric acid, 

 and mixed with the fulphuric, it afforded fulphate of lime. 



• Mineralogy of Eftner, p. 73, et. feq. f Exercit. CXVill. 



% Beoba£t und entdck, &c, Berlin, 1789, vol. III. page 336. 



D. Th« 



