390 Wells and Hurlburt—On the Ammonium-Cuprous 
120°. By means of a large prism the mean indices of refrac- 
tion were determined with a fair degree of accuracy. 
Although the results are somewhat meager, the authors are 
perhaps warranted in plotting some of them into a curve, using 
the percentages of FeO as ordinates and the divergence of the © 
optical axes, 2V, measured in degrees over the axis 6, as abscis- 
sas. This has been done for yellow light only, and it would 
indicate an interesting relation, for with an increase of FeO 
over 35 per cent 2V would probably become 180°, that is the 
material would be again uniaxial, but negative with ¢ as the 
optical axis, and with a still greater increase in FeO the optical 
axes would have to open up into the pinacoid 010. The plane 
of the optical axes therefore for the lithiophilite end of the 
series is 001, for intermediate varieties somewhat nearer tri- 
phylite it is 100, and nearer the triphylite end we may expect 
it to be O10. 
-——Axial plane 001.——-———_-—_——_—_-——- Axial plane 100, ——-— —————_——_, 
40 20 0 20 40 60 80 100 120 140 =160 180 
Laboratory of Mineralogy and Petrography, 
Sheffield Scientific School, July, 1895. 
ArT. XLI.—On the Ammonium-Cuprous Double Halo- 
gen Salts; by H. L. WeEtts and E. B. HuRLBurt. 
THE existence of ammonium-cuprous double halides has long 
been known, but since no complete investigation of these com- 
pounds had been made, a careful study of them has been 
undertaken. 
Mitscherlich* prepared the potassium salt, 4K Cl. Cu,Ol,, and 
mentioned the corresponding ammonium salt. This salt, 
4NH,Cl.Cu,Cl,, has been obtained in the present investigation. 
Deheraint described three double chlorides, 4NH,Ol.Cu, 
Cl,.H,O, 2NH,Cl.Cu,Cl, and NH,Cl.Cu,Cl,.. The first of 
these salts, if the molecule of water is omitted, corresponds to 
the compound mentioned by Mitscherlich which we have 
* Ann. Chim. Phys., xxiii, 384. + Comptes Rendus, lv, 808. 
