Mr Haidinger's Notice respecting Trona. 325 



difficult to detect the error, which becomes small, when the 

 coal-gas is of the best quality and highly purified. The me- 

 thod of Count Rum ford, is equally false with that which I 

 have now examined, particularly when the colours of the 

 flames are different, as it will be found quite impossible to 

 bring the shadows to the same density or even the same colour 

 at any distance whatever. The celebrated question which has of 

 late agitated not only the philosophical, but even the com- 

 mercial world, has not yet received a solution sufficiently ac- 

 curate to command the assent not only of the impartial ob- 

 server, but even that of rival companies. 



XXVIII. — Notice respecting Trona, the native Carbonate 

 of Soda from Fe::::an. By William Haidixg^s, Esq. 

 F. R. S. Edin. Communicated by the Author. 



Ix order to establish the grounds upon which I believe the 

 present notice not to be without interest to mineralogists, 

 I shall previously give the description of Trona itself, and of 

 the two species of hemiprismatic and prismatic Natron-salt, 

 the two latter as contained in Professor Moll's Treatise on 

 Mineralogy,* before entering upon those observations, which 

 will naturally offer themselves in the course of comparing 

 these species with each other. 



1 . Trona. 



Hemiprismatic crystals observed similar to Plate VIII. 

 Fi<>\ 5, of which Fig. 6 is a projection upon a plane perpen- 

 dicular to II and T. 



Inclination of n on n — 132° 30'. 

 of M on T = 103° 15'. 

 of n on T = 103° 45'. 



The6e angles were taken with the reflective goniometer, but 

 particularly the last of them will perhaps allow of some cor- 

 rections, if in future better crystals should be obtained. The 

 angle a, b, c, at which, in the projection, Fig. 6, the edge be- 

 tween n and n is inclined towards the face T, was valued at 

 alwmt 6'2 by the assistance of the common goniometer. There 



• Translation, vol. ii. p. 27 and 2«). 



