MONOCLINIC DOUBLE SELENATES OF THE IRON GROUP. 
397 
limit of existence is actually attained in the iron group when the potassium salt is 
reached, at a temperature which is but very slightly above 0° C. 
By intensive work on the four cold days succeeding the very cold nights in 
question—commencing early in the morning, immediately after removing the crystals 
from their mother liquor and drying them with a soft handkerchief—and by 
keeping the room cold, it has been found possible to measure completely (l) the whole 
of the exterior angles of six crystals ; (2) to determine the density of a considerable 
number ; (3) to measure the extinction angles and (4) the optic axial angles, and (5) to 
determine all three refractive indices directly, for the usual range of the seven wave¬ 
lengths of light employed by the author. The only observations, of the author’s 
usual scheme of work, which have not been possible with this salt are those carried 
out at higher temperatures, which are obviously out of the question with a salt of 
such extreme instability at temperatures even somewhat lower than the ordinary. 
At 20° C. to 30° C. decomposition is immediate, and even when working in an 
unpleasantly cold room the life of the crystals does not extend to a second day, even 
if they be covered by an inert liquid such as benzene, a device which, however, does 
somewhat prolong their existence, possibly by a couple of hours. On the second 
day the crystals were as opaque as plaster of Paris. One crystal employed, however, 
which was of tabular form and mounted between truly plane glass plates in a quickly 
drying medium of very hard Canada balsam dissolved in benzene, in a manner which 
entirely enveloped the crystal (all the rest of the air film between .the two glass plates 
being filled up by the balsam) remained transparent for nearly a week. Other 
attempts at such preservation have not, however, succeeded. Thus 5 to 6 hours’ time 
was the usual limit for work on any crop of crystals. To leave any particular crystal 
in the mother liquor until ready for work with it proved useless, for the edges became 
rounded as the temperature of the room inevitably rose slightly during the advance of 
the morning. The actual temperature of the laboratory, however, never rose above 
6° C. during all the measurements and determinations, except during the density deter¬ 
minations, when absolutely fresh crystals were employed, surrounded by the usual 
mixture of methylene iodide and benzene, which preserved them completely free from 
decomposition during the short time which the Retgers immersion method demands. 
This work now described would obviously have been quite impossible in summer. 
The grinding of section-plates and prisms was avoided as far as possible, because 
ground surfaces become opaque from decomposition with great rapidity. Special 
optical methods were employed for most of the work, the crystals being immersed in 
an inert liquid, cedar oil, which most fortunately happens to possess almost the same 
refractive index as the mean index of the crystals ■§■ (a + (3 + y), and likewise a very 
similar dispersion. But the results, although there could thus be no doubt of their 
accuracy, were confirmed on the last of the four days by the successful grinding 
of a sufficiently transparent 60°-prism from one of the larger crystals obtained, so 
orientated as to afford direct values for the indices a and y, and by the accomplishment 
3 G 2 
