524 PROCEEDINGS OF THE AMERICAN ACADEMY. 



by Jannasch and Locke, * namely solution in nitric acid with addition of 

 mercuric oxide, precipitation with ammonia and determination of phos- 

 phoric acid, most of the lime and the other bases in the precipitate, 

 while the rest of the lime and the alkalies were determined in the first 

 filtrate. Water was determined directly by fusion with plumbic oxide 

 mixed with potassium di-chromate. 



Fluorine was determined by the method of Fresenius, that is by 

 heating the finely powdered mineral, mixed with previously ignited 

 quartz, in a flask with strong sulphuric acid and absorbing the SiF 4 

 in weighed tubes with the prescribed precautions. From the total 

 weight obtained there was subtracted a correction for the general gain 

 in weight of the absorption tubes due to the action of the air current 

 on the rubber connections; etc., which had been previously determined 

 by experiment. The process was continued for five hours or to a con- 

 stant weight. Chlorine was absent. 



The mineral was soluble without residue in nitric acid. At about 

 320° C, the purple color disappears and the mineral becomes colorless 

 or faintly yellow ; this change is accompanied by some decrepitation, 

 by phosphorence, and the production of a vapor (in part water?) which 

 is deposited in drops on the walls of the tube ; there is also a petroleum- 

 like odor. The loss of weight accompanying this change was deter- 

 mined by gently heating three grammes of the mineral in a bulb tube 

 in a current of dry air, weighing, and heating again cautiously iu the 

 current of air to complete decolorization, and determining the loss of 

 weight. 



Optical Properties. f 



.- 



For the determination of the indices of refraction one of the best clear 

 crystals was used, having a deep purple color and a brilliant basal plane. 

 The determination was made with the Abbe crystal refractometer by the 

 differential method J and for this purpose a glass prism was selected 

 having the index re Na r= 1.6326, for which the boundary of total reflec- 

 tion was carefully determined and the telescope clamped. The apatite 

 crystal was then placed with its base on the glass hemisphere of the 

 apparatus and the angular difference in the boundaries for w and c 

 determined by the millimeter screw reading to six seconds. The boun- 



* Zeit. anorg. Chemie, 7, p. 154; also Jannasch, Praktischer Leitfaden d. 

 Gewichts Analyse, p. 259. 

 t By J. E. Wolff. 

 t C. Viola, Zeit. Krystall., 30, p. 438, and 32, p. 311. 



