84 Dr J. H. Gladstone on some Substances 



It was while investigating the laws of chemical affinity, 

 that I had occasion to make many mixtures of iron salts, and 

 I observed that several of these exhibited, at the edges of the 

 solution, a peculiar blue dispersion of light, similar to that 

 which occurs in bisulphate of quinine and some other sub- 

 stances, and which, since the admirable research of Professor 

 Stokes, has received the appellation " fluorescence." 



Ferrocyanide of Iron in Oxalic Acid. — If ferrocyanide of 

 potassium be added to a salt of the sesquioxide of iron, we all 

 know that a blue precipitate will form ; but if this addition be 

 made in the presence of oxalic acid, it is not a precipitate, 

 but a blue solution that results. On the surface, and about 

 the edges of this, appears a blue, which is easily distinguish- 

 able from the colour of the solution. It can be shown to best 

 advantage when the citrate of iron is used in its production, 

 since a portion of that salt always remains undecomposed. and, 

 by its green colour, renders the superficial blue more evident, 

 especially if it be in considerable excess. If there be not 

 sufficient oxalic acid, an extremely fine precipitate diffuses 

 itself throughout the liquid, and the blue refraction is more 

 apparent ; if the oxalic acid be in very large excess, the so- 

 lution is perfectly clear and transparent to transmitted light, 

 and it is difficult to observe the blue refraction by reflected 

 light, unless a ray from the sun passing through a slit in the 

 shutter be allowed to fall upon it. A solution of prussian 

 blue in aqueous oxalic acid exhibits the same phenomenon. 



It was important, of course, to ascertain whether this blue 

 appearance was due to actual fluorescence, or was merely a 

 case of opalescence produced by minutely divided solid matter. 

 Upon the slightest inspection of a solution in not very strong 

 oxalic acid, it was evident that there was a large amount of 

 opalescence ; therefore, a perfectly clear solution was pre- 

 pared by the addition of a very large quantity of the vege- 

 table acid, and the following experiments were made. 



1st, A strong solution of bisulphate of quinine was prepared, 

 and placed in a glass vessel behind a slit in the window 

 shutter, in such a manner as to cause the sun's ray to pass 

 through it. It was found to cut off the fluorescent rays so 

 completely, that a solution of bisulphate of quinine in a test- 



