338 Dr. J. H. Gladstone on Chroma/ic Phenomena [Feb. 6, 



the general rule ; yet a solution of yellow chromate of potash 

 appeared scarcely any paler when diluted with perhaps twenty times 

 its bulk of water. Sometimes also a complete change of colour 

 takes place ; thus acetate of chromium, which was red, became 

 green when considerably diluted with pure water : a few drops of 

 cochineal, stirred up in a tall champagne glass filled with water, 

 imparted a red tint to the upper wide portion, and a lavender tint 

 to the lower and narrow portion. A neutral solution of litmus is 

 blue, alkalies render this (as is well known) still more blue, boracic 

 or carbonic acid changes it to a wine red, and other acids to a 

 bright red : yet slightly acid litmus was exhibited of a pale purple 

 hue, and alkaline litmus of a deep red colour. All these phenomena 

 were stated to be dependent, not on any chemical action exerted by 

 the water, but on the quantity of the colouring substance traversed 

 by the light in its passage to the eye ; the same solution appearing 

 of different colours according to the thickness seen through, and a 

 deep stratum of a dilute liquid having the same tint as a shallow 

 stratum of the same liquid when strong. The speaker added, that 

 this phenomenon had been fully described and explained by Sir John 

 Herschel, who termed it Dichromatism, but that fresh instances of 

 it were being constantly observed ; indeed, after investigating some 

 cases of it last summer, he had, during a tour on the continent, 

 noticed a fruit sauce which constantly appeared at the hotel dinners 

 in Bavaria and other parts of Germany, and was beautifully di- 

 chromatic, red and blue, with every intervening shade of purple. 

 By this character he had traced its composition, and found the 

 colour was due to the deep red cherries which were very abundant 

 at that season. He had noticed the phenomenon likewise in some 

 specimens of the ordinary wine, in essence of lavender, in the syrup 

 of green-gage tart, as well as in some pure chemical substances, 

 such as red prussiate of potash, meconate of iron, purple comena- 

 mate of iron, citrate of iron, sulphindigotic acid, and permanganate 

 of potash. 



The prism reveals the origin of all these chromatic phenomena. 

 It shows that the different rays of the spectrum are capable of 

 penetrating different distances into a coloured medium. Thus, if 

 port wine be placed in a wedge-shaped glass vessel, and this inter- 

 posed in the refracted rays in such a position that each coloured 

 ray can fall upon the different thicknesses of the liquid, it will be 

 found that all the rays of the spectrum can penetrate a thin 

 stratum, but that as the liquid increases in depth all are absorbed 

 except the least refrangible red. Hence the thin film of a bubble 

 of port is colourless. If yellow chromate of potash be examined in 

 a similar manner, it is found to cut off the blue and violet rays at 

 once, and to transmit the less refrangible half of the spectrum with 

 equal freeness whether the stratum be thin or thick. Hence it is 

 that dilution scarcely diminishes the colour of this dissolved salt. 

 If a wedge of cobalt-blue glass (which is dichromatic) be inter- 



