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third its volume of phenyl- thiocarbimide. The refractivity is, of 
course, slightly lowered thereby, but the mixture remains liquid at 
a temperature at least as low as — 12° C. 
The coefficient of dispersion of methylene di-iodide is (/* — p H = ) 
■y > «• 
0 *062, nearly the same as that of phenyl-thiocarbimide, and decidedly 
higher than that of carbon disulphide. 
8. Solution of sulphur in methylene di-iodide. — When heated, 
methylene di-iodide dissolves a considerable quantity of sulphur, 
but much of this crystallises out on cooling. Sufficient, however, 
remains in solution at ordinary temperatures to raise the index of 
refraction of the liquid to 1*778 (at 16° C.) for yellow sodium light. 
9. Solution of phosphorus in methylene di-iodide. — This solution 
seems worthy of special attention as a substitute for the solution in 
carbon disulphide sometimes employed. I find that methylene di- 
iodide dissolves phosphorus very freely, in fact it will dissolve its 
own weight of the element without reaching the point of saturation. 
The refractivity of the light yellow solution thus obtained is, as 
might be predicted, very high indeed. I find the index of refraction 
for yellow sodium light to be 1 * 95 * at 18 J C., and the coefficient 
of dispersion is (/i H — yu H = ) 0 * 092. 
. y a 
The solution is far safer and pleasanter to deal with than phos- 
phorus itself or its solution in carbon disulphide. I have poured some 
on filter paper and left it freely exposed for hours and even days, 
without its catching fire. I have left some similarly exposed in a 
watch-glass for a week without inflammation taking place. The 
scarcely inflammable solvent seems to form a kind of varnish over 
the phosphorus which protects the latter from rapid oxidation. Still 
the precaution of having a basin of water and a wet cloth at hand 
should not on any account be neglected in dealing with the solution. 
Light acts strongly and inevitably on this medium, as on phos- 
phorus itself and other preparations containing it, causing the con- 
version of the phosphorus into the allotropic form known as “ red 
phosphorus,” which is insoluble and nearly opaque. Hence such 
substances can only be considered available for temporary purposes, 
where their very high refractivity enables critical points of structure 
to be made out with greater distinctness. In the dark they keep 
fairly well, but a slight red deposit is liable to form. 
I have now brought before the Society the principal substances 
which I have examined at present. None of them, it will seen, come 
up to a high standard of excellence as mounting and cementing 
media; but I still hope that I or others may discover something 
* I do not give the third decimal place owing to a remarkable and not easily 
explicable difficulty which occurs in getting a sharp image of the spectrometer slit 
through the solution. The same difficulty was noticed by Dr. Gladstone and Mr. 
Dale in examining the refractivity of solutions of phosphorus in carbon disulphide. 
(‘ Philosophical Magazine,’ [4] xviii. p. 30.) 
