S. L. Penfield and G. 8. Jamieson — Tychite. 221 



2*380, and de Schulten determined that of the artificial salt 

 as 2-377. By using two of the faces which meet at the apex 

 of the octahedron as a prism, we have succeeded in determin- 

 ing the index of refraction of the last crystal found. The 

 surfaces of the octahedron were not very perfect, and had to 

 be covered over for the most part, taking the reflections of the 

 signal from only the tip end- of the crystal, and the refraction 

 of light through the same. The value obtained, w y = 1*508, 

 compares favorably with that of the artifical salt, 1*510, espe- 

 cially when it is taken into consideration that the condition did 

 not favor exact determinations in either case. A further argu- 

 ment for the identity of tychite and the artificial salt, if any is 

 needed, is that at Borax Lake both tychite and northupite occur 

 together, and were formed undoubtedly under similar condi- 

 tions, while in the laboratory either of these closely related 

 chemical compounds may be made by only varying the condi- 

 tions of the experiment by using sodium sulphate for the one 

 and sodium chloride for the other. 



Of the four specimens of tychite thus far found, three have 

 been very symmetrically developed octahedrons, but small, 

 measuring not over 3 mm in diameter, and noticeably whiter 

 than the average of the northupites. It is the small size of the 

 crystals which favored the discovery of the new mineral, for 

 in the original preliminary test one of the smallest and whitest 

 specimens was selected, both because of its evident purity, and 

 also with the idea of not using up any more material than was 

 necessary. Those who may happen to have northupite crystals 

 and wish to search for specimens of the new mineral, may 

 look for tychite therefore among the smaller crystals. "We 

 are informed in a recent letter from Mr. Northup that the 

 chances of finding additional crystals of tychite, or of the asso- 

 ciated minerals, northupite and pirssonite, are too remote to be 

 seriously considered, as the old borax works are now disman- 

 tled. Tychite, therefore, promises to be a very rare mineral, 

 unless a new locality for it happens to be discovered. The 

 single crystal which we recently had the good fortune to find, 

 Mr. Foote has generously presented to the Brush Collection of 

 the Sheffield Scientific School, and both for this gift and for 

 the interest he has taken in assisting us in our investigation we 

 take pleasure in expressing our most sincere thanks. 



Comparison of tychite and northupite. — The two minerals, 

 found so intimately associated with one another and both crys- 

 tallizing in octahedrons, are chemically closely related, but in 

 order to show the relation it is necessary to double the formula 

 of northupite, as determined by Pratt. The compositions 

 may then be expressed as follows : 



