AND ON THE ACTION OF LIGHT ON BODIES. 449 
usual, and then allowing the plate to remain for a long time over 
it. If it be wished to engrave such pictures, the fixed ones would 
be found well-adapted for the purpose. It is not necessary to 
remove the iodine by means of hyposulphite of soda, because it 
may be taken away just as well by rubbing. It might appear 
as if the mercury in the fixed images not only adhered, but 
entered into combination with the silver, which, however, is by 
no means the case, as will presently be seen. 
If the mercury be now heated still stronger, the plate acquires 
a yellow appearance, and above 120° R. the image becomes 
negative, i.e. the bright parts of the object appear dark, and 
vice versd. If further heated, this negative image becomes fixed ; 
_and if it be rubbed we are soon convinced that the mercury has 
left those parts to which it previously adhered, for these spots 
appear quite bright; on the contrary, it now fastens on those 
places where it formerly did not. If these negative images have 
become fixed, it is difficult to get them off the plate; even fluids 
and sharp powders do not always effect it ; and it often happens, 
‘that after other processes, as for instance exposure to vapours 
of chlorine, the traces of the old picture reappear in the plainest 
manner. 
These peculiar phenomena, exhibited by the vapours of mer- 
cury, explain the difficulty with regard to the condensation of 
-yapours, for they take place with all of them. Place, for instance, 
a screen with characters cut in it over a polished plate, and 
breathe over it; take the screen away and allow the breath to 
evaporate, and breathe on it again; those parts which before 
were affected by the breath will now appear black. The experi- 
ment may be repeated again and again, and always with the 
same result. Now breathe longer and more strongly; the cha- 
racters will then generally appear lighter than the surrounding 
parts; and the image is usually destroyed at the same time, for 
only traces of it are seen afterwards. 
That the vapours of iodine have an analogous effect might be 
supposed from what precedes, and it will be hereafter proved 
when I come to consider these vapours more in detail. 
If this behaviour of the vapours by continued action should 
appear strange, I have to remark that it differs in no respect 
from that of light itself, and I therefore now turn to perhaps the 
most interesting proposition in this department,—that the same 
modification is produced in plates when vapours are condensed 
