12 



GIBBS. 



These observations are more or less .correct, depending upon the light 

 by which the crystals are viewed. By reflected light the appearance is 

 usually a brilliant, blue-violet, while by transmitted light the predominat- 

 ing color is yellow to reddish-brown. Even these shades vary greatly 

 under the microscope. From the constitution of the compound, as 

 developed by Nietzki and Schmidt, one naturally suspects that its color 

 will be more apt to be of the quinone order and that the description 

 yellow-brown to red-brown is most appropriate. Some difficulty was 

 experienced in obtaining the crystals in their true form, probably due 

 to slight impurities. By washing many times with boiling alcohol, 

 cold alcohol, boiling ether and cold ether consecutively, after recrystalli- 

 zation from aniline, quite pure crystals were obtained. I am indebted 

 to Dr. W. D. Smith, chief of the division of mines, of this Bureau, for 

 the following description of the crystals : 



The compound crystallizes in the tetragonal system and occurs in very thin 

 plates, generally under a millimeter in length. The usual forms are prism, 

 pyramid and base, diamond and square. In mass and by reflected light these plates 

 have a decided purple hue. By ordinary transmitted light many of them are 

 quite colorless. The crystals are trichroic. The accompanying fig. 1 gives the 

 form, and figs. 2 and 3 show the pleochroism : 



«9?30 



n.ih yil 



F 'G. 1. 



Rfrid.il> brow 



Fig. 2. 



* GrMfliih y .11.- 



Fig. 3. 



These plates show parallel extinction in polarized light. The index of re- 

 fraction is medium and the double refraction is low. First order colors. 



The melting point of the compound is stated by Nietzki and Schmidt 

 to be above the thermometer scale, while Zincke and Hebebrand say that 

 it does not melt and sublimes without decomposition. I have found 

 that the crystals melt readily and recrystallize in broken and imperfect 

 forms on cooling. Probably slight decomposition takes place. The melt- 

 ing or softening begins at 338° and the purest samples obtained were 

 completely melted at 342°, uncorrected. A nitrogen determination of this 

 substance gave 9.76 per cent, while the theoretical value is 9.65 per cent. 

 In concentrated sulphuric acid the coloration is stated by Knapp and 

 Schultz to be "fuchsin red," and by Nietzki and Schmidt "violet." I 

 have observed that the former statement more accurately describes the 

 color. 



