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



beetroot-red by gaslight. Under like circumstances the powdered 

 substance is of the same colour. Minute crystals, examined in daylight by 

 the microscope, are of a pale green tint ; fragments of large crystals coarsely 

 powdered appear of almost all tints, rich green and blue prevailing. 

 Larger crystals, deposited by spontaneous evaporation of a cold solution, 

 appear of two colours chiefly, a beautiful blue and rich green, with some- 

 times a tinge of deep red, their appearance being exceedingly charming. 



Occasionally crystals of a good size, seen by reflected light, appear of two 

 colours ; this has been observed in a mass of crystals removed from a 

 glass dish, where they had grown in contact with the glass ; distinct blue 

 and green patches of colour were seen intermingled. These crystals 

 most likely had not true surfaces, but were covered with smaller crystals, 

 so that light reflected from the large facets -became colour by transmission 

 through small crystals. A quantity of the salt crystallized in isolated 

 prisms in the bottom of a beaker glass was examined by looking directly 

 at the sun ; thus viewed they were ruby-red ; on turning away from the 

 direct rays at an angle of 30° they appeared green ; and on moving still 

 further round, some of them appeared blue. We may thus vary the 

 colour of the crystals by admitting white light to them at different 

 angles. An experiment was made in order to ascertain whether viewing 

 a crystal through different axes produced alterations in colour. It was 

 not easy to see how this could be done, because a crystal of an inch 

 thickness would certainly be too large to transmit light sufficiently well. 

 I devised, however, the following plan, and successfully overcame the 

 difficulty. Taking a quill tube of good glass free from streaks and 

 bubbles, I drew this out sharply to a capillary size, and ran a minute 

 crystal into it with a little of the mother-liquor ; by blowing into the tube 

 the liquid was slowly expelled, while the crystal was securely wedged 

 into the fine end, and so remained stationary when the tube was turned 

 over. To make a stage for holding this, a glass slip had a cork bored 

 with a hole cemented to it ; the tube being passed through the hole, could 

 be turned over without displacement from the field of the microscope. 



In the first observation the crystal looked opaque ; but here and there 

 an indistinct red light shone through on turning it ; a green tint with a 

 shade of red next made its appearance, the green became full and bril- 

 liant; opacity next succeeded, and then blue with a tinge of green; 

 finally the full beautiful blue colour. In each revolution of a crystal 

 there were two periods of blue, two of green, and two of opacity with a 

 tinge of red. When a crystal so small as to be quite transparent in any 

 position was examined in this manner, it was seen that purely red light 

 was transmitted through its edges. It is thus rendered very evident that the 

 play of colour is due to pleochroism, or the property which, by virtue of 

 double refraction, some minerals possess of transmitting light of dif- 

 ferent tints through their different crystalline axes. I have examined 

 many well-crystallized salts in the manner already described, and there 

 are none which can compare with potassio-calcium chromic oxalate. The 



