1873.] Mr. W. N. Hartley on a new Chromic Oxalate. 505 



jpale yellow, green, and blue in daylight, and through all shades of pale 

 yellow, pale orange, red, and blue in candle-light." I find the colour of 

 small crystals by reflected light is not always blue, but varies between 

 blue and green, just as does the new salt under the circumstances already 

 mentioned. Again, the variations in tint of small crystals is attributed 

 to the differences in thickness ; but it is really due to pleochroism. of a 

 character identical with that in my salt. A crystal powdered up coarsely, 

 and placed under the microscope, no longer presents a blue colour, but 

 consists of a beautiful collection of coloured fragments, decided red, green, 

 blue, and yellow, besides all imaginable intermediate tints, such as neutral 

 green and greenish blue. These tints change as the fragments roll over 

 in the liquid surrounding them. This property is not noticed ' by 

 Haidinger, who, in a contribution to Poggendorff's ' Annalen ' * on the pleo- 

 chroism of this potassium salt, states that the dichroiscope showed the 

 ordinary image to be green, and the extraordinary blue, the blue tint 

 being the brighter, so that, the colours being brought together in day- 

 light, the blue prevails. 



Brewster noticed previously that, the images — produced by double re- 

 fraction in these crystals, the least refracted was blue, and the most 

 refracted green. 



Bunsent has obtained different absorption-spectra by passing polarized 

 light through different axes of crystals of didymium sulphate. 



Light in traversing a crystal of such a substance as this new com- 

 pound is both coloured and polarized by the crystal itself ; it is there- 

 fore impossible to regard its spectrum by other than polarized light. We 

 therefore get different kinds of absorption in different crystalline axes, 

 which are manifested in a striking manner by the varying colours the 

 salt presents. This well exemplifies the fact that pleochroism results 

 from the polarization of light traversing a doubly-refracting coloured 

 medium. 



From the microscopic examination of many similar new compounds, 

 namely ammonio-calcium, potassio-ammonium, potassio-lithium, potas- 

 sio-thallium, barium, and strontium chromic oxalates, it was found that 

 the efficiency in displaying pleochroism was inversely as the molecular 

 weight of the compounds, because the property depends upon the 

 amount of colour the chromium gives to the crystals. The proportion 

 of chromium is of course larger when it is associated with other elements 

 possessing small atomic weights. And whether we have 40 parts of 

 calcium or 406 of thallium in a compound associated with 52 of chro- 

 mium, is a matter which greatly affects the percentage of chromium in 

 the substance. 



It was at first supposed, from the -totally different appearance of the 

 salt when precipitated, and when slowly formed in large crystals, that 

 two compounds were obtained, it being a common occurrence to have 

 * Yol. lxxvi. p. 107. t Pogg. Ann. vol. cxxyiii. 



