278 Mr. R. Hunt on the Changes of Bodies in Darkness. 
I became desirous of ascertaining whether the mercurial 
plates would produce any change upon the precipitated iodide 
of silver. [find bymany experiments, that if the iodide of silver 
is pure, no more change is produced than is produced upon 
it by diffused light; but if it is rendered sensitive by a trace 
of the nitrate of silver, it is then darkened as by solar in- 
fluence. 
Sensitive iodide of silver being placed upon a plate of glass, 
a mercurial plate was fixed 3th of an inch above it. In three 
days the iodide of silver had become a deep brown, almost a 
black, and the mercurial plate was covered with the yellow 
iodide of mercury. Nitric acid dissolved the dark portion of 
the silver salt, as did also ammonia, on which was formed 
Faraday’s oxide of silver, thereby proving the change, either 
by a primary or a secondary process, of the iodide into the 
oxide of silver. This experiment has been repeated at least 
a dozen times, and always with the same results, If a little 
heap of the iodide of silver is placed under a mercurial plate, 
it is exceedingly interesting to witness the gradual formation 
of the very beautiful coloured rings on the mercury in the 
progress of its conversion into an iodide. By prolonged ac- 
tion the yellow iodide passes into the bright red biniodide of 
mercury. I have some experiments now in hand, which con- 
vince me that similar chemical changes are to be effected 
through considerable spaces. I have succeeded in decom- 
posing the iodide of copper and the iodide of gold by mer- 
curial plates placed nearly a quarter of an inch above them. 
I have an extensive record of results similar to those I have 
now detailed, all of them showing that the changes brought 
about by this mysterious agent, whether it be heat, light, or 
an undiscovered element, cannot be referred to those rays 
which the admirable researches of Sir John Herschel have 
shown to be the operative ones in producing the photographic 
pheenomena which have so interested the world by their novel 
beauty, and which Professor Draper includes within his ge- 
neral term—tithonicity. With regard to the detithonizing in- 
fluence of the gases mentioned by Dr. Draper in his paper 
in your March Number, I can only consider the results, which 
I find to be as he has stated, as the simple reconversion of 
the decomposed iodide of silver into another definite chemical 
compound. An iodized plate is exposed to light, the iodide 
of silver or other sensitive salt is decomposed, and in a state 
to receive mercurial vapour. It is now passed through an 
atmosphere of iodine, of chlorine, of bromine, or of nitrous 
gas. Chemists are well aware of the surprising energy with 
which these bodies attack the metals, consequently the ex- 
