Mounting Media of High Befractivitg. Bg H. G. Madan. 277 
and mercury in the hope of removing this if present. I have also re- 
distilled it under greatly reduced pressure (-^ atmosphere), and 
filtered it through animal charcoal, but the yellow colour, though 
much reduced in intensity, still remains. 
5. Phengl-thiocarbimide (formula of molecule, C 6 H 5 CNS), a 
substance belonging to the class of mustard oils, one of the many 
discoveries of Dr. A. W. Hofmann.* I need not go into the details 
of its preparation, which are of more interest to chemists than to 
physicists, especially as I find that it can now be obtained commer- 
cially from Schuchardt, of Gorlitz (English agents, Messrs. F. E. 
Becker and Co., 33 Hatton Wall, London, E.C.). 
It is a perfectly colourless liquid, practically unalterable by light 
(I have exposed some to sunlight for a whole summer without more 
than a trace of yellow colour appearing). It has a characteristic but 
not disagreeable smell like that of ordinary mustard oil ; when warmed, 
however, rather pungent vapours are given off. Its boiling point is 
very high (222° C.), and it is not readily inflammable, — in fact, a 
lighted match may be thrown into it without setting it on fire. 
I have determined carefully the indices of refraction for the prin- 
cipal Fraunhofer lines. For sodium light, /^ D = 1 * 654 at 10° C. ; and 
its dispersive power is rather higher than that of carbon disulphide, 
viz. (/bi H — fji H = ) 0 • 060. These high refractive and dispersive powers 
x a 
are attributed by Dr. Gladstone f to the presence of the phenyl radicle 
and the sulphur atom in it, both of which have a very high specific 
refractive energy. 
It is evident from what has been said that phenyl-thiocarbimide 
possesses all the good qualities of a-monobromonaphthalene and of 
carbon disulphide, without the yellow colour and doubtful permanency 
of the first, and the dangerous inflammability and volatility of the 
second. I have used it in a prism belonging to my spectrometer for 
many years, and found it very satisfactory in every way, giving 
excellent definition (especially in the more refrangible part of the 
spectrum), and being much less liable to disturbance by internal con- 
vection currents than carbon disulphide. I have also made a polarising 
prism on Jamin’s principle, — a thin plate of Iceland spar placed 
obliquely in a cell filled with phenyl-thiocarbimide, — and found it to 
answer well ; though of course a solid prism is preferable, as long as 
pieces of Iceland spar of sufficient size can be obtained. 
Phenyl-thiocarbimide readily dissolves meta-cinnamene, forming, 
when equal weights of the substances are taken, a viscous mass like 
Canada balsam, having an index of refraction about the mean of the 
two constituents (/a D = 1*63 at 15° C.). This does not harden so 
readily as balsam, but in all other respects it seems excellently fitted 
for use in mounting microscopic objects. 
* ‘ Proceedings of the Royal Society,’ ix. p. 25o. 
t ‘ Journal of the Chemical Society,’ xlv. p. 241. 
