506 Mr. W. N. Hartley on a new Chromic Oxalate. [Nov. 27, 



two modifications of a chromium salt, as for instance the blue and the 

 green solutions of chromic nitrates and sulphates. Chemical analysis, 

 however, showed them to be identical in composition, while the crystal- 

 lization and close examination of many specimens disclosed the nature of 

 the substance. I have come to the conclusion that this is certainly the 

 most remarkable example of pleochroism at present known. 



Note. — Eeceived November 7, 1873. 



I find that the potassio-calcium chromic oxalate is a substance which 

 excels uranium compounds as a test for the purity of white light. It is 

 well known that uranic salts exhibit dark , bands in the blue part of the 

 spectrum, which cannot be seen by gas-light although plainly visible by 

 day ; they are, however, shown with some distinctness by the lime-light. 

 This chromium compound shows a red tint by gas-light, red and blue by 

 the lime-light, while by the light of magnesium wire or the sun it displays 

 red, blue, and green. So sensitive, indeed, is this substance that its true 

 colour cannot be seen on an ordinary November morning in London, some 

 crystals appearing blue, others of a colour varying between neutral grey, 

 lilac, and red tints, according to the yellowness of the atmosphere. 



To the list of substances examined for pleochroism, I have now to add, 

 besides the minerals well known to possess this property : — 



Sulphate of iodo-quiuine. 



Sulphate of iodo-cinchonidine. 



Sulphate of iodo-quinidine. 



Potassio-palladium chloride. 



Ammonio-palladium chloride. 



Potassium permanganate. 



Teriodide of tetrethylammonium. 

 The powerful polarizing qualities of the first two iodine compounds 

 are well known ; they have been described by Dr. Herapath* and Prof. 

 Stokes. As they crystallize, however, in thin plates, it is impossible to 

 transmit light through more than one axis; they cannot therefore be 

 brought into comparison with my substance. Endeavours to obtain 

 large thick crystals did not meet with success. Sulphate of iodo-quinidine 

 crystallizes in beautiful orange-brown prisms with a violet lustre ; it is, 

 however, but a feeble polarizer, and therefore does not exhibit pleochroism 

 to any noticeable extent. The double palladium salts which have been 

 previously examined are beautifully pleochroic prismatic crystals ex- 

 hibiting an orange-brown tint along the principal crystalline axis, and a 

 green in other positions. It is well seen in crystals \ millim. in thick- 

 ness. Potassium permanganate is a substance which, seen even in 

 minute crystals, is still opaque. A very dilute solution was allowed to 



* Proc. Roy. Soc. viii. 1856-57. Phil. Mag. S. 4. toI. iii; 1852 ; iv. 1852 ; vi. 1853. 



