are capable of undergoing in Darkness. 273 
it very sensitive to luminous agency. If we take this sensi- 
tive iodide and allow it to blacken, and then throw it into di- 
luted nitric acid, all the darkened portion will be dissolved, 
and a pure non-sensitive iodide left behind; or if we throw 
it into ammonia the darkened part will disappear, leaving the 
yellow salt, and in a few hours the ammonia will be covered 
with a pellicle of Faraday’s oxide of silver. 
According to Dr. Draper, if a properly prepared plate is 
exposed to light until in a fit state to receive the mercurial 
vapour and then placed in a dark room for four or five hours 
with a blackened metallic plate suspended one-eighth of aninch 
from its surface, we find, when it is exposed to the vapour of 
mercury, that those parts only are attacked by it which corre- 
spond with the suspended screen. It is inferred from this, “ that 
the tithonicity that had originally disturbed the surface of the 
plate equally all over has escaped away from those portions 
that were uncovered ; but that its escape has been entirely pre- 
vented by the action of the screen, and this must be through 
radiation... And further, that the rays that do thus escape away 
are absolutely invisible to the eye.” Dr. Draper then pro- 
ceeds to point out the analogy between this case and one of a 
body cooling by radiation, concluding, ‘the two cases are 
absolutely alike. Tithonicity therefore radiates exactly after 
the manner of heat.” When we can account for the effects 
produced by known causes, we are not justified in seeking 
for new ones. Inmy paper on Thermography *, I have shown 
that any blackened body placed above an iodized or a simply 
polished plate of metal,—whether that plate has been exposed 
to light or not is of no consequence,—will produce such a 
change as will dispose the covered portion to receive the 
vapour of mercury. If I can succeed in proving that all the 
phznomena described by Dr. Draper are to be produced as 
well on simple metals as on surfaces of the ioduret of silver 
or other sensitive surfaces, and equally in perfect darkness as 
in the brightest light, and that many of them are to be traced 
to chemical change, I shall certainly convince you of the im- 
propriety of attributing them to the chemical, or, as Dr. 
Draper terms them, the tithonic rays. 
As it was difficult, from the length of time required, to 
produce any decided effects upon the surfaces of polished 
metal plates by the solar spectrum without a good heliostat 
to keep the sun’s image stationary for some hours, I was 
obliged to have recourse to different absorptive media in. my 
endeavours to ascertain whether any of the rays of Licur 
‘ * Phil. Mag., S. 3. vol. xxi. p. 462. 
Phil. Mag. 8. 3. Vol. 22. No. 145, April 1843. = T 
