Mounting Media of High Bef r activity. By II. G. Madan. 275 
monoclinic prisms. The associated water of crystallisation is readily 
given off at temperatures between 120° and 130° C., and the crystals 
break up into a white powder, which, on being heated to a temperature 
of 170° C., melts into a nearly colourless liquid. At 200° C. the 
liquid begins to darken in colour, owing to incipient decomposition 
and liberation of carbon ; hence in dealing with the substance as a 
“ mountant ” it is important not to raise the temperature more than 
necessary above the melting point. On cooling the liquid it solidifies 
into a light yellow resinous mass, having so slight a cohesion and 
so low a conductivity for heat that a block of it cracks up into small 
fragments under a slight stress or change of temperature, almost like 
unannealed glass ; nor does very slow cooling appear to prevent this. 
I have also tried the effect of quick cooling by pouring liquid quini- 
dine into cold water, but the only result seemed to be to make it more 
like a “ Rupert’s drop ” than before. 
When this resinous solid is again heated gradually, it changes 
into a mass of interlacing prismatic crystals and rosettes, forming 
a very beautiful polariscopic object. The change certainly begins 
below 90° C., but proceeds more rapidly at a few degrees higher. 
On further heating, the crystals melt, passing into the resinous 
colloidal condition mentioned above. I think it likely that the 
tendency to crystallisation complained of by those who have mounted 
objects in quinidine is mainly due to the slides having been exposed 
to a rather high temperature, such as that of a very hot room. 
Also it should be noted that the presence of a few nuclei, such as 
particles of dust or of one or two minute unconverted crystals in the 
resin, would materially promote crystallisation. It would be advis- 
able to filter the liquid, if practicable, before placing any upon the 
slide. 
The fragility and unstable equilibrium of the colloidal form of 
quinidine caused me considerable difficulty in making a prism of it 
for the purpose of determining its refractivity. I was anxious, how- 
ever, to do this, as the index of refraction was reported to be as high 
as 1*7 (for what wave-length I do not know). I am disappointed to 
find that the index for yellow sodium light is no higher than 1*802. 
Its coefficient of dispersion, moreover, is not particularly high ; 
(fi H - = ) 0 * 048 (that of a-monobromonaphthalene between the 
same wave-lengths is 0*051). 
From the above observations on the effect of heat cn quinidine, it 
would seem advisable, in using the substance as a mounting material, 
to cool the slide pretty quickly, in order to get the resin past the 
turning point of 90° before it has time to take the wrong turn. But 
in this and similar substances there must, I fear, always remain the 
inherent tendency to revert to the crystalline condition, which is 
obviously the condition preferably assumed by the molecules when 
they are free to do as they like. 
