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IV. 'A^opcjyioTa, No. I. — On a Case of Superficial Colour presented by a homogeneous 

 liquid internally colourless. By Sir John Frederick William Herschel, Bart., 

 K.H., F.R.S., 8^c. 8^c. 



Received January 28, 1845, — Read February 13, 1845. 



A CERTAIN variety of fluor spar, of a green colour, from Alston Moor, is well 

 known to mineralogists by its curious property of exhibiting a superficial colour, 

 differing much from its transmitted tint, being a fine blue of a peculiar and delicate 

 aspect like the bloom on a plum, and like that bloom might perhaps be referred to 

 a peculiar texture of the surface, the result of crystallization, were it not that it 

 appears equally on a surface artificially cut and polished. Glasses also are manufac- 

 tured which, by the agency of a delicate superficial film, consisting apparently of a 

 dull green-coloured powder, and reflecting (or rather dispersing) a green light, ex- 

 hibit a brownish red tint by transmission ; chloride of sulphur, and the infusion of 

 lignum nephriticum are particularized in some books as exhibiting different colours 

 by transmitted and reflected light. As respects the chloride of sulphur, the state- 

 ment is incorrect, and has originated in a misapprehension of its scale of absorbent 

 action, which (as is the case with many dichromatic media) causes its hue to change 

 from green to red by mere increase of thickness. In the infusion of lignum nephri- 

 ticum, and in one other instance which has occurred to my notice, the reflected tint 

 arises from suspended particles too minute, or too nearly of the specific gravity of the 

 liquid, to be separated by subsidence*, the transmitted colour being that of the trans- 

 parent liquid in which they float, and the particles themselves being opake. 



The case which I am about to describe is not precisely parallel to any of these, 

 though far more striking than either. That of the fluor spar presents the closest 

 analogy to it, though from what we know of the impracticability of obliterating the 

 internal structure of mother-of-pearl by any artificial polish, the diflference between 

 the solid and fluid states of aggregation precludes any argument from that pheno- 

 menon to the one in question. 



The sulphate of quinine is well known to be of extremely sparing solubility in 

 water. It is however easily and copiously soluble in tartaric acid. Equal weights of 

 the sulphate and of crystallised tartaric acidf , rubbed up together with addition of 

 a very little water, dissolve entirely and immediately. It is this solution, largely 

 diluted, which exhibits the optical phenomenon in question. Though perfectly trans- 



* I write from recollection of an experiment made nearly twenty years ago, and which I cannot repeat for 

 want of a specimen of the wood. I think the filtered liquid did not exhibit the double colour, 

 t Citric acid answers equally well. 



