LXI. On s&me supposed Crystals of Trona, By Henry James 

 Brooke, Esq,y F.R.S. ^c. With an Analysis by Mr.ALL4j| 

 J)iCKy Metallurgical Laboratory, School of Mines^. 



Ill WAS favoured some months since by Mr. BoUaert with some 

 crystals of this form, which were found ^ vJs ^j. 



with glauberite and hayesine at Tarapaca. 

 On comparing the angle of the edges ab with 

 that of the faces r^t of Trona, as given at p. 598 

 of the new edition of Phillips's Mineralogy, 

 the two angles were found to agree so closely 

 as to lead to the conclusion that the Tarapaca crystals might be 

 trona. They have, however, since been analysed at the School of 

 Mines, and are found to be thenardite. The faces, also, cd, dd) 

 have been measured by Professor Miller, and the angles found to 

 agree very nearly with those of corresponding faces of thenardite. 

 No faces, however, which would truncate the edges ab of the Tar^T 

 paca crystals have yet been observed on crystals of thenardite ; bt^t 

 as the indices of such faces would be 530, their occurrence is not 

 improbable. Whether this agreement of angle between particular 

 faces on crystals belonging to two different systems of crystal- 

 lization, of which a few other instances have been noticed, is 

 merely accidental, or is the result of some at present unknov^n 

 law, is a matter which requires further investigation. *^^''^ ^^^^ ^'^ 



Analysis. 



2*73 grs. were dissolved in water acidulated with hydrochloric 

 acid. Insoluble siliceous matter was separated by filtration, 

 which after ignition weighed 0*06 gr. 



Chloride of barium was added to the filtrate. The precipitate 

 of sulphate of baryta weighed 4-39. 



The excess of baryta in the filtrate was removed by carbonate 

 of ammonia, and the soda determined as chloride of sodium, of 

 which the weight was 2*18. 



1'27 gr. heated in a tube gave only a trace of water, but a 

 violet vapour was evolved which condensed into minute crystals 

 of iodine, which furnished the characteristic reaction with starch f. 



Neither nitric acid, lime nor magnesia were present in sensible 

 (j[uiantity. ^^* 



:, Results tabulated:—:;^ Oxygen. ' ^'^^^ ;^ 



y' Soda . ^.'^.^. 42-37 10-83 ^^^oxl^lA 



lo iiutri .a i s^iph^ric acid . 55-1 ] 32-98 '~ ^<> ^««^^« 



HUH giiriol fjisoiuble residue 2-19 ^^"^^'{^ <>^J 



l^mM le/JS^t'Js Nab; S03. 'f'^ >f^m.n.^V 



* Communicated by the Authors. .^..^,...,1 , ^. .v/. .= . ;,rr -u... ,\ 

 [t The evolution of iodine by heat is interesting. Various saline products 

 from the same district have been examined in the metallurgical laboratory 

 by Mr. Dick, and all found to yield traces of iodine. The peculiar odour 

 evolved by hayesine is probably due to the slow evolution of iodine by 

 exposure to the atmosphere. It seems not unlikely that a supply of iodine 

 may be obtained from the above-mentioned products. — J. Percy.] 



