512 
PROFESSOR HEXEY A. MIERS: AX EXQUIRY IXTO 
images) two bright images corresponding to faces inclined to each other at 109° 6': 
and the an^le of emergence through D was — 46° 51'. This gives the index as 
r34312. 
A third determination was maale with the same crystal on the following day, using 
the acute angle of the octahedron as prism, he., total reflection taking place at B and 
emergence through C. 
This gave BC — 70 43 and the angle of emergence 6 = — 4° 16', leading to the 
value 1’34171 for the index of the liquid; in this case, however, the crystal was 
probably not growing, but was in contact with a diluted liquid, for the line of total 
reflection was not well defined, the addition of a little water did not affect the 
reading, nnd etched triangles made their appearance on the faces. The value 1’3417 1 
IS simply file index of an almost saturated solution. Another crystal was mounted 
and examined in the same way with the obtuse angle for prism ; this gave 
a .= 109° 39i', d = - 47° 19i', p = 1-34422. 
The same crystal, examined on the following day at 14^^°, gave 
a =r 109° 30A, d = — 47° 19i', p = 1-34313. 
But the faces now yielded triple images, so that this determination is possibly not 
so reliable as the preceding one. 
The crystal was then withdrawn and the glass prism inserted in place of it; this 
gave 
a = 44° 58', d = 19° 37', p = 1 ’34263. 
The above observations are sufficient, I think, to establish conclusively that the 
liquid in contact with a growing crystal of alum is slightly supersaturated, and that 
its index differs by about ’0007 from that of a saturated solution at the same 
temperature. Other observations led to the same result. 
The mean of the best determinations for the solution in contact with a growing 
crystal at 14°, 
1-34375, 1-34331, 
1-34343, 1-34308, 1-34330, 034332, 
1-34313 is p = 1 -34336. 
The best determination for tbe saturated solution at 14° gave 
P = 1 -34263. 
Interpreting these results by the curve of Diagram I, we arrive at the conclusion 
that at 14° C. : — 
