164 Scientific Proceedings, Royal Dublin Society. 
probable value 32, we may suppose the arrangement of atoms in 
the crystal to approach to Hg—H¢g acting equatorially, and Hg-Cl 
acting vertically, we then obtain :— 
Hg,, dy = 10, Y= 10. 
Hg,(Cl, 12 = 4°99, 02 = 26°9. 
The sum is 36:9, the volume of calomel is 36:3. 
Numerous other compounds have been examined, with inte- 
resting and confirmatory results; but we must content ourselves 
here with a single example, the most interesting, however, of 
them all. This is potassium copper chloride, CuCl, 2KCl, 
2H,0, the refractive indices for the line B are n, = 1°6365, 
ne = 1:6148. 
It is obvious that no constitutional formula can be devised for 
this salt, without invoking a higher valency for some of the 
elements than they commonly possess. But this is a difficulty 
with which chemists are familiar in a large number of other 
cases, nor need it surprise us if in the solid state an element 
should exercise more numerous bonds than in the liquid or 
gaseous states; 1t were rather to be expected. There are more 
ways than one in which the components of this hydrated potas- 
slum copper chloride can be built together into the complete 
molecule: we may choose as the simplest the following :— 
Horizontal Plan. Vertical Elevation. 
H K 
u s 
H—Cl_Cu—_Cl_H B—CiCr— Cee 
d 
Hi t 
The atom of copper may be regarded as placed at the node of 
a tetragonal crystal-net, then on four rectangular ranges lying in a 
plane at right angles to the optic axis are the groups of —Cl-H, 
on two vertical ranges corresponding to the optic axis are the two 
groups -O-K; acting vertically we shall then have the con- 
stituent Cu,-O-K, which will be related to the index for the 
extraordinary ray, and acting horizontally Cu,-Cl-H, which will 
be related to the ordinary ray. 
