272 Mr. R. Hunt on the Changes which Bodies 
moon will darken several argentine preparations, but merely 
to a disturbance of the equilibrium of the caloric latent in the 
plates. It will be found by experiment that heat or cold ap- 
plied to one portion of a metal plate will render that part 
more susceptible of condensing vapour than the other parts. 
I must confess, however, that I hold my judgement suspended ; 
for there are curious circumstances which I am endeavouring 
to investigate, which appear to indicate the influence of an 
agent with which we are unacquainted. I will now come to 
the more important part of the Professor’s communication,— 
** Proof of the existence of DARK TITHONIC rays analogous to 
the rays of DARK HEAT.” 
Dr. Draper and M. Moser do not appear to admit that the 
film of ioduret of silver on the Daguerreotype plates is de- 
composed by solar agency; the former attributing the change 
of colour to a disturbance produced by the absorption of the 
**tithonic rays,” and the latter to a molecular change pro- 
duced by light ‘externally blackening the iodide or other 
compound of silver.” 
My own impressions are certainly adverse to this. I for- 
merly* expressed it as my opinion that the iodide was con- 
verted into an oxide of silver by the light. I now believe the 
oxidation to be a secondary action, due to the influence of 
the atmosphere. A great number of carefully conducted 
experiments have convinced me that the solar rays liberate 
iodine, leaving the silver in a state of very fine division loosely 
on the surface of the solid metal. Lodide of silver is insoluble 
in nitric acid or ammonia, although I find the former has the 
power of decomposing that salt by long-continued action; 
but it will be found that the darkened portions of the solar- 
ized Daguerreotype plates are rapidly dissolved by diluted 
nitric acid, and slowly by ammonia. If, as I have described 
in the paper already referred to, we receive an impression of 
the solar spectrum on an iodized plate, we may, by gently 
rubbing, remove all the darkened portion, and thus distinctly 
mark the spaces of maximum and minimum action, proving 
that some kind of decomposition has taken place. If we 
take the precipitated iodide of silver, we shall be enabled to 
trace more distinctly all that occurs, It has been remarked, 
that this salt exists in two states; the one sensible, and the 
other insensible to light}. I find that perfectly pure iodide 
of silver undergoes little or no change by long exposure, but 
the presence of the smallest portion of nitrate of silver renders 
* Phil. Mag., N.S. vol. xvi. p. 275. 
+ Sir Jobn Herschel, Phil. Trans., part 1. for 1840; Phil. Mag., S. 3, 
vol. xvi. p. 275. 
