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Transactions of the Society, 
The parallel case of sulphur will occur to the mind of everyone. 
This substance takes three allotropic forms, two of them crystalline. 
When either of these latter forms is heated to 150° C., it passes into 
a plastic colloidal condition, but can only be retained even tempo- 
rarily in this form by being poured suddenly into cold water. In a few 
days it inevitably reverts to the crystalline condition ; and the same 
change (as with quinidine) takes place quickly on heating the colloid 
to 100° C. Mr. Lewis Wright suggested to me that admixture with 
Canada balsam might prevent this ; but on trial I do not find it answer. 
8. Naphthyl-phenyl-ketone (formula of molecule, Ci 6 H 12 CO). 
This was discovered by Grucarevic and Merz,* and seemed at first 
likely to be an unexceptionable cement for polarising prisms. I pre- 
pared it by a method given in the paper referred to, which I need not 
describe here, as the details might not be intelligible without previous 
study of organic chemistry. It is a light yellow substance, viscous 
like Canada balsam at ordinary temperatures, but becoming more 
liquid when moderately heated, almost odourless, with little or no 
tendency to act on or injure other substances. The index of refrac- 
tion for yellow sodium light I found to be 1 * 669 (at 15°), almost 
exactly that of Iceland spar for the “ ordinary ” ray. If there was no 
doubt as to its permanency, it would be very valuable for the pur- 
poses both of physical optics and of microscopy. The first specimen 
which I prepared remained for months quite unchanged (although 
tested by exposure to a temperature as low as — 20° C.), but eventually 
turned into a mass of yellow crystals ; and a fine polarising prism 
made for me in Paris on M. Bertrand’s principle, in which this cement 
was used, after lasting for four or five years at least, has now become 
useless from the same cause. The ketone may be easily restored to 
the colloidal condition by re-distillation, but the change is not perma- 
nent. Hence, while I think the substance worth the attention of 
microscopists for temporarily mounting objects, my recommendation 
cannot extend further. 
4. a-monobromonaphthalene (formula of molecule, C 10 H 7 Br), dis- 
covered by Laurent f in 1835. This substance is so well known to 
microscopists that I need not say much about it. 
The index of refraction for sodium light is 1*66 (at 23° C.), and 
its dispersive power is almost exactly the same as that of carbon di- 
sulphide (ya H — ya Ha =) 0*051. It has a very high boiling point 
y 
(285° C.), and is more stable than such bromo-compounds usually are. 
Hence it would be well adapted for filling hollow spectroscope-prisms 
but for its incurable yellow colour, which implies of course loss by 
absorption of the more refrangible part of the spectrum. It seemed 
possible that this yellow colour might be due to the presence of a 
trace of free bromine, and I have digested the liquid with zinc, sodium, 
* ‘ Bericlite der deutschen cliemischen Gesellschaft,’ vi. p. 1238. 
t 4 Annales de Cliimie et de Physique,’ lix. p. 216. 
