Mounting Media of High Refractivity. By H. G. Madan. 279 
soda-glass,” behave in a similar way, becoming semi-opaque or “ de- 
vitrified ” when heated to the softening point and retained long in 
that condition, owing to the formation of crystalline silicates in the 
colloidal mass. 
7. Methylene di-iodide (formula of molecule, CH 2 I 2 ), discovered as 
long ago as 1858 by Butlerow.* 
This is in several respects a very remarkable substance, and I am 
surprised that, while it has been known and used for many years by 
mineralogists on account of its high density, t it does not appear to 
have attracted the notice of microscopists. 
It is, when pure, a pale yellow liquid with a slight sweetish smell, 
boiling at 181° C., freezing at 5° C., and hardly at all inflammable, 
at any rate at ordinary temperatures. Its density is no less than 3 * 34 
times that of water, so that nearly all kinds of glass, as well as most 
minerals (even fluor spar and hornblende) float in it. 
Its refractive and dispersive powers have been accurately deter- 
mined by Dr. J. Id.. Gladstone,! who finds its index of refraction for 
yellow sodium light to be 1 ’756 at 10 ’5° C. My own examination 
of a different sample gives 1'743 at 15° C., — a very close agreement 
when the correction for decrease of refractivity with temperature is 
made. This, it will be observed, is exceptionally high, in fact the 
highest of any single organic substance I have met with, and induces 
me to recommend methylene di-iodide (which is easily procured com- 
mercially) as a temporary mounting medium. I regret that I must 
say “ temporary ” only ; for like many other iodides it has a slight 
tendency to decompose, especially when exposed to light, with libera- 
tion of iodine, which colours the liquid red, and would act upon and 
damage some delicate tissues and crystals. I find that after one 
hour’s exposure to direct sunlight the liquid becomes perceptibly 
darker, and four hours’ exposure turn it a light orange about the 
tint of “ golden sherry.” Another day’s exposure to ordinary day- 
light only deepens the colour slightly, and after a -week’s exposure 
the liquid is still only light red, a tint hardly perceptible in thin 
films. In the dark it may be kept for years without alteration, and 
the free iodine may at any time be readily removed by shaking up 
the liquid with dilute solution of potassium hydrate (“ caustic pot- 
ash ”), allowing it to stand, decanting off the potash, and removing 
the last traces of moisture by leaving a lump of calcium chloride in 
the liquid for a short time. Most of the colour may be removed 
more simply, but more slowly, by placing a strip of zinc in the 
liquid for a day or two. 
One drawback to the use of methylene di-iodide is that it freezes, 
as already mentioned, at a point a little above the freezing point of 
water. This, however, can be prevented by adding to it about one- 
* Comptes Rendus, xlvi. p. 595. 
t ‘Neues Jahrbuch fur Mineralogie,’ 1886, ii. p. 72; 1888, i. p. 213; 1889, ii. 
p. 185. X ‘ Journal of the Chemical Society,’ lix. p. 293. 
