470 PROFESSOR LUDWIG MOSER ON LATENT LIGHT. 
advance facts as objections to that correct statement, that the con- 
densation of vapours has an action similar to that of light ; while, 
if we consider them from the proper point of view, viz. that the 
various kinds of vapour contain different coloured latent light, 
we find in them the strongest confirmations of the proposition. 
When I first discovered the effect of light in the precipitation of 
vapours, [commenced a series of experiments to find whether any- 
thing analogous to colour occurred in these experiments. I then 
believed that the different degree of elasticity might perhaps be 
compared to the different colours of the spectrum; but this 
opinion was overthrown, when it was found that there was no 
effect produced by mercurial vapours of high tension which 
could not be brought about by the long-continued action of va- 
pours of a lower degree of tension ; consequently in the case of 
the vapours of mercury there was nothing in their degree of 
elasticity that could be compared to the colours of the spectrum. 
There would have been no reason for ascribing to this species 
of vapour a peculiar action different from others; but it was 
more natural to suppose that in all cases a continued action 
would have the same effect as a high degree of tension. Only 
those vapours like those of water could make an apparent ex- 
ception, inasmuch as they do not remain on the plates, but eva- 
porate again, and thereby give occasion to the production of a 
double process. The question remained, therefore, unanswered, 
until I obtained an insight into the action of the condensation 
of vapours, as produced by the light which was in a combined 
state. 
It was now not to be doubted that each species of vapour re- 
tained a peculiar kind of light in combination, either rays of a 
simple colour, or of several, but combined in a definite pro- 
portion. After this point had been settled, I soon became con- 
vinced that what we as yet know of the action of the different 
colours of the spectrum is quite insufficient for determining with 
safety the latent colour of even one vapour. Our knowledge is 
not insufficient because too few substances have been examined 
with regard to their behaviour towards light; on the contrary, 
I believe that there is still sufficient to be studied in one sub- 
stance, viz. iodide of silver ; and I know only one question, to be 
hereafter mentioned, which I do not think can possibly be an- 
swered by iodide of silver alone, but requires the test of other 
kinds of matter. I shall therefore principally make use of this 
