420 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



of the crystal corresponds to a direction of minimum velocity. More- 

 over, the pleochroism is interchanged, so that now: 



a axis = violet. 

 c axis = lavender. 



The changes in the indices of refraction and the birefringence 

 undergone by the green variscite crystals on changing to the lavender 

 phase are also very marked. The mean refractive index has dropped 

 from 1.560 to about 1.448 and the birefringence from 0.032 to 0.003 

 or possibly even less. The birefringence does not exceed 0.003 in any 

 section and is much less in most cases. The approximate values of 

 the indices, determined by oil immersion, are as foUows: 



«r = 1 .447, /? = 1 .448, r = 1 .450 

 ^-«; = 0.003. 



This decrease in the values of the refractive indices and the bire- 

 fringence is accompanied by the loss of water of the mineral, and it is 

 a fact worth emphasizing that the loss of water, without destroying 

 the optical possibilities of the mineral, decreases both these values. 



When the lavender crystals are heated with water on the steam 

 bath (not over 100° C.) for several days, they gradually lose their 

 lavender color and become a dirty grayish white and are then non- 

 pleochroic. A sample, treated in this way for about a week, was then 

 air dried for several days and the loss on ignition ( = water absorbed) 

 determined. This amount was found to be 2.05 per cent. The ignited 

 crystals had again become deep lavender in color. It therefore 

 seemed possible by appropriate treatment to change the dehydrated 

 lavender crystals back to their original vivid green color. With this 

 view in mind, some of the lavender crystals were treated with hot 

 water for several days untU they became grayish white in color and 

 then sealed in a glass bomb with a little water and heated for several 

 hours at a higher temperature. The glass bomb was first heated for 

 four hours at 170° C, the next day for the same time at 190° C, and 

 the third day for a similar length of time at 225° C. Unfortunately it 

 was not possible to heat the tube at these temperatures continuously, 

 as should probably have been done. After three days' heating, as 

 just described, the grayish white crystals had become very pale 

 greenish yellow, but when the crystals were removed from the tube 

 and the loss on ignition determined after they had been air dried 

 for several days the result was disappointing. A loss of only 1.12 

 per cent was found, so that the crystals had absorbed less water in the 

 glass bomb treatment than by simply treating with hot water. 



The crystals from the glass bomb were very light brown or nearly 

 colorless when viewed under the microscope. Pleochroism was dis- 

 tinct, though very slight. Parallel to the c axis, a direction of maxi- 



