PROFESSOR LUDWIG MOSER ON LATENT LIGHT, 469 
the limits of what has been empirically proved) if we have a va- 
pour of a certain elasticity and temperature, and any plate also 
of a determinate temperature, the condensation of the vapour is 
not found at once, but it depends upon those actions of light to 
which the plate has been exposed. This state of the matter is 
not sufficiently advanced to prove what influence light has in 
the change of the state of aggregation. Although a silver plate, 
e.g. has the same temperature in every part, still the aqueous 
vapours will be condensed, either in a greater or less degree, by 
certain portions of it, according to the action of light which these 
parts have undergone*. 
The same holds good with regard to the condensation of the 
vapours of mercury, iodine, chlorine, &c.; it depends as much 
upon light as upon heat, and it is evident, from the difference of 
those vapours which I have examined, that such a result is of 
general application. For instance, although aqueous vapours 
are precipitated very readily on plates of silver, they evaporate 
again just as quickly, and exhibit very small or no adhesion. 
The vapours of mercury adhere permanently, for the images of 
Daguerre do not exhibit any perceptible change even after a 
length of time. But what is very peculiar in this respect is, 
that they do not seem to combine with the mass of the silver, 
or with that of gold, or even zinc, while mercury in a fluid form 
exhibits so great an affinity for these substances. Vapours of 
iodine combine chemically with the silver. Notwithstanding, 
then, that these vapours behave so differently in certain respects, 
they still agree together in one point, viz. that their condensa- 
tion is influenced by light just as much as by heat. Indeed we 
should have been able to draw this conclusion from the Da- 
guerre’s pictures alone, if we had had a correct insight into the 
process which produces them. 
I will now proceed to the interesting question concerning the 
colour of latent light. The determination of this point is as 
important as it is difficult, and I have only succeeded after 
many attempts with some kinds of vapours, but in these cases 
in a manner that promises sufficient security. This examina- 
tion is important, inasmuch as one might easily be induced to 
« With regard to the greater or less degree of condensation, I refer to my 
first treatise, in which I have examined the vapours of mercury in this respect 
with more minuteness. The vapours of water do not differ in this from those 
of mereury, and when of sufficiently high tension invert the images just as well 
as the others. 
