190 M. C. Lea—Reactions of Silver Chloride and Rromide. 
as much chlorine as the normal white chloride. _ Yet the loss of 
chlorine has been found too small to be weighed. Fresenius 
doubts if a loss in weight could be detected by the most delicate 
balance, and Von Bibra in the investigation above referred to, 
could not find the slightest loss in weight. . 
With a view to obtain some quantitative indication in the 
matter, the following determination was made. 
Silver chloride was precipitated with an excess of hydro- 
chloric acid, was well washed, and exposed to bright sunlight 
for five days. During this time it was spread in a very thin 
layer over the bottom of a large white porcelain basin, was fre- 
quently stirred up to bring constantly new surfaces to the 
light, and was kept moistened with water. 
Of the resulting dark powder two grams were taken and 
were thoroughly treated with sodium hyposulphite to remove 
the unaltered chloride. Previous experience had shown that 
extraordinary precautions were necessary to effect this thor- 
oughly, as the removal of the last portions of normal silver 
chloride is very difficult. Accordingly the strong solution of 
hyposulphite was many times renewed, each time being left to 
act for from twelve to twenty-four hours. Finally the gray 
residue (metallic silver) was washed, dried and weighed, an 
found to amount to twenty-one milligrams. 
It thus appears that as the result of five days’ action of 
strong sunshine, with frequent stirring up and mixing to bring 
fresh portions to the light, about one per cent only of the silver 
chloride was acted upon. And if we suppose this action to con- 
sist in removing one-half the chlorine, then the whole loss 12 
weight by the action of the light should be but little over one- 
tenth of one percent. This proportion is of course not inap- 
preciable, and the observations of Fresenius and of V. Bibra 
above quoted must be taken as referring to shorter exposures. 
It was mentioned that another difficulty in verifying the 
nature of the action of light lay in the fact that those sub- 
stances which can “is or dissolving out the unaltered 
chloride, also unfortunately attack the altered substance. 
The two reagents most effectual for this removal are sodium 
ts s The facts here mentioned lead to the curious reflection that the permanency ¢ 
ordinary : prints is greatly diminished by the fixing process. FOF 
