AND ON THE ACTION OF LIGHT ON BODIES. 443 
I now introduced these plates into the vapours of mercury, and 
then the perfect image became visible ; that is to say, Daguerre’s 
phznomenon was produced without the intervention of light, for 
the experiments succeeded just as well by night as by day. Other 
similar plates, which after being iodized did not exhibit any 
figures, were laid in open day- or sunlight, when the represen- 
tations of the objects employed quickly made their appearance, 
and indeed depicted with the greatest delicacy. I cannot pos- 
sibly describe here all the experiments that I made on this sub- 
ject; I should by so doing far exceed the limits of this paper. I 
will only mention that the vapours of chlorine and chloride of 
iodine perform the same service; that the phenomena may be 
produced on blackened iodide of silver, on copper, steel, &c. 
although I was not capable of giving the latter body any very 
good polish. I have also used platinum and black mirror glass 
for experiments of this kind, not only with the vapours of water, 
but also with those of mercury, which are condensed and adhere 
to them. 
The result of these experiments is of considerable importance. 
The discovery of Daguerre consists, in a physical point of view, 
in the knowledge of the fact that the action of light on iodide of 
silver produces a state in which it is capable of condensing the 
vapours of mercury, which adhere to those parts affected. This 
peculiar action of light stood so far quite isolated. Here we have 
the same action, aly much more general: by the contact of aclean 
or polished surface with a heated body, or vice versd, by the 
contact of a heated surface with a cold body, particular parts of 
this surface are brought into such a condition that they are ca- 
pable of condensing all kinds of vapours, and of causing them to 
adhere. And it must be remarked, that even when the vapours 
combine chemically with the substance, as for instance iodine 
with silver and copper, this state produced by contact makes 
itself equally visible ; partly by a different colour, which proves a 
different degree of combination ; partly so, that when exposed to 
light or the vapours of mercury, the image becomes clearly de- 
veloped. In fact, when a plate has been in contact with a body 
it may be afterwards exposed not only to vapours of iodine, but 
also to those of chloride and bromide of iodine, and yet the image 
is produced in light, or in the vapours of mercury. 
The action of light was therefore imitated and extended by my 
experiments; and indeed, as it appeared, by the employment of 
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