PROFESSOR LUDWIG MOSER ON LATENT LIGHT. 467 
ture; consequently, in the case of light, a similar definition is 
of more practical use, and the condensation of vapours is a good 
means of proving the existence of some action of light, even 
although it cannot be employed in its measurement. If, then, 
this test for light, as well as another hereafter to be mentioned, 
is considerably inferior to that for heat (the thermometer), 
still the former offers, as we shall see, advantages of a peculiar 
kind, which are quite wanting in the instrument for the mea- 
surement of caloric. 
In now turning to the real subject of the treatise, I will refer 
to my previous experiments, which have in so many ways esta- 
blished the fact, that the condensation of vapours produces the 
same effect as the direct action of light. Even some peculiar 
effects of light, as for instance the blackening of the iodide of 
silver, may be produced by the condensation of mercurial va- 
pours, of which I have mentioned several examples. I only re- 
fer to this phenomenon in order to add that I have obtained 
the same effect with the same vapours with nitrate of silver, by 
means of papers brushed over with a solution of this salt; and, 
moreover, to add, that this very striking observation is by no 
means new, but was made several years since, although no im- 
portance was ever attached to it, nor could well be, inasmuch 
as the phenomenon acquires its true interest only when con- 
nected with other facts. Bayard observed that when papers 
covered with bromide or chloride of silver are taken out of the 
camera obscura before a trace of a picture is visible, and are 
then treated with vapours of mercury, those parts which were 
exposed to the light become black. 
If the precipitation of vapours produces effects similar to those 
of light, we are justified in assuming a latent light belonging to 
the vaporized state, just as from an evolution of heat under 
similar circumstances we adopt a latent state of that power. 
For the present all that remains to be shown is, how the reversed 
process, evaporation, produces effects of light; and this I have 
observed on evaporating water, alcohol and zther. I employed 
plates of silver, gold, copper, German silver, and glass; we may, 
however, evaporate fluids of any superficial extent, if they only 
produce small permanent changes, and we are able to write down 
as it were the effects which have taken place. Experiments of 
this kind, to succeed well, are by no means easy of execution; 
even distilled water readily leaves traces on well-polished bodies, 
