396 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
of ‘‘The Optical Properties of the Cyanurets of Platinum and Magnesia, and of 
Barytes and Platinum,” salts which he had received from M. Haidinger of ^ ienna. 
The notice is chiefly devoted to the properties of the reflected light ; but with 
respect to the latter of the salts, Sir David remarks that “it possesses the property 
of internal dispersion, the dispersed light being a brilliant green, while the trans- 
mitted light is yellow!' Although the distinction between true internal dispersion 
and opalescence was not at the time understood, there could be little doubt from the 
nature of the case that the internal dispersion mentioned by Sir David Brewster 
was, in fact, an instance of the former of these phenomena ; but I could not try for 
want of a specimen of the salt. Some months ago I received fiom nl. Haidixger a 
specimen of the first of the salts mentioned at the beginning of this paragraph, 
namely M. Quadrat’s cyanide of platinum and magnesium, a salt of great optical 
interest on account of the remarkable metallic reflexion which it exhibits. On 
examining the salt, I was greatly interested by finding that it was highly sensitive, 
the fluorescent light being red. This induced me to form some of Gmelin’s platiiio- 
cyanide of potassium, and I found that the blue light which this salt exhibits in 
certain aspects is, in fact, due to fluorescence, a property which the salt possesses in 
an eminent degree. Having afterwards received some of the same salt pure from 
Professor Gregory, I applied it to the formation, on a small scale, of the platino- 
cyanides of calcium, barium, strontium, and two or three others. The three salts 
last named, of which the second is that mentioned by Sir David Brewster, are all 
eminently sensitive, the fluorescent light being of different shades of green. It is 
only in the solid state that the platinocyanides are sensitive ; their solutions look 
like mere water. The precipitates which a solution of platinocyanide of potassium 
gives with salts of the heavy metals are, in most cases that I have yet observed, in- 
sensible. With a solution of pernitrate of mercury, however, a bright yellow pre- 
cipitate is produced which is exceedingly sensitive, so as to look brighter than even 
yellow uranite. The light emitted is yellow. It forms a very striking experiment to 
place side by side on the tablet with the second combination a drop of a solution of 
platinocyanide of potassium, and another of pernitrate of mercury. Ihe drops 
look like water on the dark field of view ; but when they are mixed, a precipitate is 
produced which glows like a self-luminous body with a yellow light. The pre- 
cipitate with nitrate of silver is also sensitive, though not in so high a degree. Ihe 
platinocyanides are extremely interesting ; first, as forming a third case, or rather 
class of cases, in whieh the property of fluorescence is attaehed to substances 
chemieally isolated in a satisfactory manner (though I believe chemists are acquainted 
with a few other organic compounds to which the property belongs), the othei 
two cases being salts of quinine and of peroxide of uranium ; and secondly, as exhibit- 
ing a new and remarkable feature, which consists in the polarization of the fluorescent 
light. 1 content myself at present with this notice ; the salts require a more ex- 
tended study. 
