36 M. C. Lea—Combination of Silver Chloride 
volatilizes, and the portion so treated becomes permanently 
low 
If one of these cards be exposed to sunlight with a portion 
perfectly protected, the exposed part becomes slightly darkened. 
This effect takes place with an exposure of half an hour or less 
and is scarcely increased if the exposure be prolonged for man 
hours. By keeping for a few days in the dark, the deepening 
of color gradually fades out until the exposed parts are not to 
be distinguished from the rest. This effect may be repeated 
several times with the same card. 
In reference to the compound of silver iodide with mercuric 
iodide, I feel constrained to differ from Meusel, who considers 
it only a mixture and not a combination. I take it to be a 
compound, though an extremely loose one, and for the follow- 
ing <a pais which apply equally to the silver chloride com- 
ou 
n 
So long as the substances are only mixed, and before they 
have united, the characteristic color of the mercuric iodide is 
always conspicuously observable, and no mechanical mixing, 
however thorough, is capable of concealing it. The eventual 
disappearance of this color seems to be a proof of combination. 
This view is farther strengthened by what takes place in the 
case of silver bromide. When Hel is placed in contact with 
AgBr, no combination takes placeas in the case of AgCl and Agl 
under similar conditions. Hven after many days the color of 
the mercuric salt is conspicuously visible. In one case I kept 
HglI in contact with excess of AgBr, both freshly precipitated, 
for several weeks under water, at the end of which time the 
particles of HgI could be oe under a lens with per- 
fect facility. At the end of weeks, the mixture of AgBr and HgI 
retains exactly the appearance which it presents when first 
mixed (supposing, of course, that it has been protected from the 
light). AgCl and AgI, when mixed with Hgl, present at first 
the same appearance, which, however, subsequently changes 
completely, and a substance with new properties is the result. 
is seems clearly to indicate a combination. 
e striking fact that the thermochromic properties of mer- 
curic joaiis are inverted in the new compound, which passes 
from yellow to red by heat, instead of from red to yellow, 
and the far greater sensitiveness to heat of the new substance, 
are strong arguments of combination. 
Meusel’s explanation = the — of color, in his view of 
mere mechanical mixture, makes it depend on a diminished 
power of absorbing red pad: when warmed, which he ascribes 
to silver iodide. But the experiment which I have described 
with the substance a on eard-board dieprotes this explana- 
tion in two distinct ma 
At a temperature about out 30° or 40° of Hubreoheit above the 
