MONOCLINIC DOUBLE SULPHATES CONTAINING AMMONIUM. 
41 
Murmann and Rotter obtained indirectly values for the /3 index varying from 
1‘494 for red light to 1'502 for blue light. 
General Formula for /3 (corrected to a vacuum):— 
o i-io a n 904 205 7 045 250 000 000 
O = i 4oUy + ---i-r .... 
X" X 4 
The a indices are also reproduced by the formula if the constant 1‘4809 is 
diminished by 0'0096, and the y indices if it is increased by 0'0048. 
Observations at 70° indicated that for 55° rise of temperature the refractive indices 
are diminished by about 0'0027 for y, but only by 0'0005 for a, and by an inter¬ 
mediate amount for j3, the index approaching nearer to the midway position 
between the a and y indices. 
The refractive indices of the four salts containing copper are compared in the next 
table. 
Comparison of the Refractive Indices. 
Index. 
Light. 
KCu 
sulphate. 
EbCu 
sulphate. 
NH 4 Cu 
sulphate. 
CsCu 
sulphate. 
f 
Li. 
1-4807 
1 • 4858 
1-4878 
1-5017 
C. 
1-4811 
1-4862 
1-4883 
1-5021 
a j 
Na. 
1-4836 
1-4886 
1-4910 
1-5048 
T1. 
1-4861 
1-4912 
1-4938 
1-5074 
i 1 
F. 
1-4893 
1-4943 
1-4971 
1-5108 
l 
G. 
1-4944 
1-4991 
1-5024 
1-5159 
f 
Li. 
1-4834 
1-4878 
1-4972 
1-5032 
C. 
1-4838 
1-4882 
1-4977 
1-5036 
13 
| 
Na. 
1-4864 
1-4906 
1-5007 
1-5061 
T1. 
1-4889 
1-4933 
1-5035 
1-5089 
F. 
1-4922 
1-4966 
1-5067 
1-5123 
l 
G. 
1-4975 
1-5013 
1-5121 
1-5174 
f 
Li. 
1-4990' 
1-5007 
1-5020 
1-5122 
C. 
1-4994 
1-5011 
1-5025 
1-5126 
i 
r • • • • <! 
i 
Na. 
1-5020 
1-5036 
1-5054 
1-5153 
T1. 
1-5047 
1-5064 
1■5083 
1•5180 
! 
F. 
1-5081 
1-5098 
1-5116 
1-5216 
i 
i 
G. 
1-5134 
1-5148 
1-5171 
1-5266 
Mean refractive 
index X (a + t3 + y) 
1-4907 
1-4943 
1-4990 
1-5087 
for Na light 
Double refraction, Na y _ a .... 
0-0184 
0-0150 
0-0144 
0-0105 
It will be clear from this table that the refractive indices of ammonium copper 
sulphate are slightly higher than those of rubidium copper sulphate; the mean 
refractive index at the foot of the table shows this well. The double refraction is 
also only slightly different from (beyond) that of the rubidium salt, being slightly 
less, this property being a diminishing one while refractive index is an increasing- 
property as the alkali-metal series is ascended. 
tol. ccxvi.— a. o 
