AND ON THE ACTION OF LIGHT ON BODIES. 453 
is, as has been shown, in the same state as iodide of silver that 
has been nearly blackened by light, it is evident that vapours of 
low tension must produce those states, which, in the case of 
iodide of silver, precede the blackening, and which cause a 
stronger condensation of the mercury; and not only vapours of 
low tension, but also those of a high degree, supposing they act 
only for a short time; for it is always the same with vapours as 
with light, the same effect is produced by high as by low inten- 
sities, provided the times are proportionate. I therefore repeated 
the former experiment, by exposing a well-polished plate with a 
prepared screen to the vapours of mercury, but only for a short 
time. Nothing was visible when the plate was taken out, but 
it was in the same state as an iodized silver plate that has re- 
mained in the camera obscura the proper time. After it had 
been again exposed to the mercurial vapours, but free and with- 
out the screen, an image of the excised figure was produced, 
and, as was to be expected, the previously uncovered parts were 
lighter, and condensed more mercury. The picture acquired 
strength by repeatedly warming the mercury, as is often seen 
in Daguerre’s pictures, and in general in those images produced 
by aqueous vapour. Continued breathing on these latter gene- 
rally causes them to appear clearer and better defined, because 
the vapours of water, like all others, continue the action on the 
surface which has been begun, and are capable, in the course of 
time, of inverting it. ' 
In the preceding experiment the mercurial vapours were em- 
ployed twice; the first time in so weak a state, or for so short 
a period, that they produced no perceptible precipitate. Other 
vapours, as for instance those of iodine, may be employed with 
equal success, instead of those of mercury, in the first instance. 
Let vapours of this kind act on particular parts of a plate of 
silver for so short a time that no colouring of the plate is pro- 
duced, about the fourth of that which is necessary to produce 
the first yellow tinge. If it be now introduced into the mercu- 
rial vapours, the mercury is deposited on those parts which had 
been previously affected by the iodine. 
It will be evident, from the above detailed experiments, that 
the vapours of iodine act on silver just like light; and it is in 
fact a matter of indifference whether light be first allowed to 
act on the plate of silver and then iodized, or whether it be 
iodized first and then exposed to the influence of light; but 
