1873.] Mr. W. N. Hartley on a new Chromic Oxalate. 503 



platinocyanides, most beautiful crystalline coloured bodies, attracted my 

 attention ; but their prominent properties are the converse of my salt ; 

 they reflect two or three different coloured rays, but transmit only one. 

 Such salts are : — 



Magnesium platinocyanide Mg Cy 2 . Pt Cy 2 . 7H 2 O. 



Barium platinocyanide Ba Cy 2 . Pt Cy 2 . 7H 2 0, 



Potassio-magnesium platinocyanide .... Mg K 2 . Cy 4 . 2Pt Cy 2 . 7H 2 O. 

 Barium chloro-platinocyanide 5(PtCy 4 ) Ba Pt Cy 4 Cl 2 .ccK 2 O . 



The following I have also examined : — 



Green salt of Magnus 2KH 3 . Pt Cl 2 , 



Eoseo-cobaltic chloride Co Cl 3 . 5H 3 N . H 2 O, 



and Claudet's salts Co Cl 3 . 5H 3 N . Hg Cl 2 . 



Haidinger*, however, who examined the magnesium platinocyanide, 

 found that although to the unaided eye the light transmitted was of a 

 pure carmine-red, yet the dichroscopic lens showed that the colour of 

 thin crystals was distinctly separated into crimson-red and a tint less 

 blue. The two axes of thin crystals give spectra which do not differ 

 from each other materially, nor from the solution of this salt in par- 

 ticular, nor from chromium salts generally. I noticed that solutions of 

 the salt, when left to evaporate spontaneously in porcelain dishes, fre- 

 quently formed crystalline crusts, which adhered to the sides for an inch 

 or so above the liquid ; and it occurred to me that such crusts would 

 serve admirably for spectroscopic observations, isolated crystals being 

 too small. Having purposely prepared a good quantity of the salt in 

 this state, portions were selected, mounted, and examined with the micro- 

 spectroscope in brilliant sunlight, with the result that three different 

 spectra were observed. The accompanying sketch shows the relative 

 positions of the transmitted rays. 



Eed 



G-reen .. 



Blue 



The red tint, seen when looking at the diagonal to the base of the 

 prism, consisted of one red luminous band only between and 2 on 



* Pogg. Ann. vol. xvii. p. 89. 



