SULPHATES. 



79 



FERROUS SULPHATE (FeSC4+7H 2 O). 

 (Curve b, fig. 62.) 



This crystal dehydrated so rapidly that the surface became rough. 

 There is one well-defined maximum at 9.1 /* which is harmonic with the 

 SO 4 band at 4.55 p. 



ALUM (K 2 S(XA1 2 (SO4)8+24H 2 O). 

 (Curve a, fig. 63.) 



Aschkinass (loc. cit.) found a small reflection band at 9.05^. The 

 present examination was made to compare it with that of other com- 

 pounds having the SO 4 radical. The maximum was found at 9.1 /*. 



6 6 



FIG. 63. Alum (a) ; Anhydrite. 



IO 



12/A 



ANHYDRITE (CaSO 4 ). 

 (Curve b, fig. 63.) 



Cleavage pieces pasted upon cardboard to form an area sufficiently 

 large for reflection. In comparison with selenite, it will be noticed that 

 the large maximum of the latter is here found at 8.6 ju, followed by a 

 small band at 9.1 p. It seems a little surprising that the latter band 

 should be so small if it is a harmonic of the large band (of CaSO 4 ) at 



4-55 /* 



DIASPORA AIO(OH). 



(Cleavage piece parallel to b. Curve a, fig. 64.) 



This is the most remarkable mineral yet examined. For the region 

 up to 8 p. it has no marked absorption bands. Beyond this point condi- 

 tions are entirely reversed. There is a broad reflection band extending 



