164  Onthe Detithonizing Power of certain Gases. 
Without multiplying the description of these experiments 
further,—for the ingenuity of any one who repeats them will 
suggest many modifications which may give rise to striking 
results, —I will in conclusion give the reasons which have led 
me to suppose that in all these phenomena two different 
principles are engaged,—vapour action, and radiation. 
I have stated that the ELecrro-NEGAtIvE bodies possess 
this detithonizing quality in a very marked manner. I do 
not wish it to be understood, however, that there is any rela- 
tion of antagonization between that particular class of sub- 
stances and the tithonic rays. It appears to me that their 
peculiar quality, in the circumstances described, may be traced 
to the fact, that silver, an electro-positive substance, happens 
to afford the sensitive surface. 1 have however prepared a 
paper which takes up the consideration of the conditions 
and theory involved, and will not at present anticipate what 
has to be offered when that paper shall be published. 
The action, then, which these different gases and vapours 
exhibit, is so intense as to mask the feebler effect of radiation. 
Thus it takes several hours’ exposure in the dark, and after a 
long subsequent process of mercurialization, to prove the true 
radiant effect,—a slow detithonization, which could be brought 
about by vapour action iz an instant. But whoever has seen 
the symmetrical or rather geometrical lines that are left, when 
the slower process is followed, must be struck with the per- 
suasion that the phenomenon he witnesses is obeying geome- 
trical laws, and is not due to the irregular action of a dilute 
and varying current of vapour. 
Thus, on repeating carefully the experiment cited at the 
close of my last paper, in which a lens is laid on a tithonized 
surface and left in the dark, I found that after the mercurial 
process was completed, the plate exhibited a dark central spot 
surrounded by a white annulus. On drawing upon paper a 
section of the lens and the sensitive surface, I found that a 
line drawn from the extreme edge of the white annulus to the 
edge of the lens was a tangent to the lens at that point; that 
a line drawn from the extreme edge of the central dark spot 
would, after reflexion by the convex surface of the lens, be 
found precisely on the edge of the white annulus; the edge 
of the annulus and the edge of the spot thus having a true 
catoptrical relation to the curvature of the lens. 
Now, although in laboratories such as that in which my 
experiments are conducted, the vapours of these different 
electro-negative bodies are unquestionably present, and may 
produce a part of the phenomenon witnessed, yet inasmuch 
as that phanomenon follows laws that are apparently of a 
