ELISHA MITCHELL SCIENTIFIC SOCIETY. 65 
mately, Using the Bertrand ocular, this was found to 
be 33° frem one pinacoid on to the other over the twinning 
plane. The axis a makes an angle of 52° with the verti- 
cal axis C in the obtuse angle B. 
The double refractien is positive and weak. The acute 
bisectrix ¢ is at right angles to the pinacoid 010, and the 
divergence of the optical axes is large. 214 probably 
varies from 120° to 130°, but this could not be measured 
directly. 
Chemical analysis.—The mineral was purified for 
analysis by means of the heavy solution and that which 
was used varied in specific gravity from 2°278 to 2°360. 
The analyses were made by Mr. H. W. Foote,** who 
describes the methods used as follows: 
‘Water was determined by loss on ignition, and silica 
and alumina by the ordinary methods after fusion with 
sodium carbonate. The filtrate from the alumina pre- 
cipitation was evaporated with aqua regia to remove the 
large excess of ammonium salts and a small amount of 
ammonium chloride was again added. Calcium, barium 
and strontium were then precipitated together, with a 
considerable excess of ammonia and ammonium carbo- 
nate, and magnesia was determined in the filtrate. The 
mixed carbonates were dissolved in’ kydrochloric acid 
evaporated to dryness and taken up in about 300°° of wa- 
ter. The method used for separating barium was that 
recommended by Fresenius.* To the hot solution, a few 
drops of acetic acid were added and 10°° of a 10 per cent 
solution of ammonium chromate containing a_ small 
amount of dichromate. After standing until the solu- 
tion became cold, the clear liquid was decanted and the 
precipitate of barium chromate was washed with a weak 
chromate solution and with water. The precipitate was 
*Zs. Anal. Chem., xxix. 426. 
**Of the Chemical Laboratory of the Sheffield Scientific School, 
New Haven, Conn. 
