Power of certain Gases, &c. 163 
In several successive trials I found that the magnitude of 
this spectre diminished as the time of exposure to the vapour 
had been prolonged. _ 
Next, I repeated the same trial, using the plate and lens 
as just described ; but immersing the plate in the vapour of 
bromine instead of that of iodine,—a still more remarkable 
image emerged on mercurializing. This image, like the former, 
was circular and black, but all round it for a certain space 
there was an annulus of narrow dimensions of pure unmer- 
curialized silver, the deep black of which contrasted strikingly 
with the blue black of the spectre, and its outer circumference 
was marked by a faint whitening of the plate,—faint, but as 
sharp as it is possible to conceive. ; 
In a third trial things were conducted as before, except 
that now chlorine, diluted with atmospheric air, was used; 
the spectre again came out, and did not differ in any obser-~ 
vable manner from that produced by iodine. 
In a fourth trial the vapour of nitrous acid was used as a 
detithonizer. In this case the edges of the spectre commonly 
had a gradually shading outline, and only in one instance did 
I find that sharpness of termination exhibited in the other 
cases, 
We therefore perceive that iodine, bromine, chlorine, and 
nitrous acid can detithonize a surface on which light has 
fallen: they can undo what the tithonic rays have done. 
In repeating these experiments, as for example the one by 
iodine, if the common iodine-box be used to effect the detitho- 
nization, two or three seconds of time is all that is required, 
A longer period is demanded when the vapour is very weak, 
but when strong the effect is almost instantaneous. 
This detithonization and production of spectral images can 
therefore be accomplished in an incredibly short space of time. 
I made trials with other substances, such as hydrogen gas 
and the vapour of liquid muriatic acid. The former to a cer- 
tain extent, though not near so powerfully as the electro-nega- 
tive bodies mentioned, could produce the change in question ; 
the latter seemed to be without any perceptible action. 
To the list, with the other electro-negative substances, I be- 
lieve oxygen ought to be added ; for, on repeating the same ex- 
perimentand raising thetemperature of the plate in atmospheric 
air so as to maintain the tithonized surface at about 200° Fahr, 
for several minutes, a certain effect which in an imperfect 
way resembled those already described was exhibited. Oxy- 
gen, therefore, diluted as in atmospheric air, at 200° Fahr., 
may be regarded as possessing to a small extent the property 
in question. 
. M2 
