AND ON THE ACTION OF LIGHT ON BODIES. 447 
deepened figures of an engraved stone, appeared lighter or darker. 
If they appeared dark the vapours were precipitated exclusively 
or for the most part on the touched parts, and the contrary if 
they appeared white. The phenomenon seemed, however, to 
be complicated and irregular; for sometimes, by means of the 
breath, I obtained light and dark figures, and sometimes both 
together; and finally, from one and the same figure, at first a 
white image, and on breathing stronger a dark one, and then 
again a white one. Mercury exhibited similar complicated ap- 
pearances ; it was deposited principally either on the touched or 
the untouched parts; oftentimes it could be wiped off, at others 
it was dry, at least it could not be removed by rubbing; and, 
finally, the vapours of iodine were irregular in the extreme. 
The parts of the silver that had been touched, appeared, after 
iodizing, sometimes darker and at other times lighter than the 
untouched parts. When brought into the light, either of them 
first became blackened. Once, when brought into the light, the 
letters of a seal were at first black, then the surrounding parts 
became darker and the letters lighter; then again the phzno- 
menon was reversed, and the letters were darker, and remained 
so for several days. This inversion may be easily explained 
from what has already been said concerning the alternating ac- 
tion of light, and would be comprehensible if the deep figures 
of an engraved stone, which lay upon the same plate as the seal, 
had exhibited the same phenomena; but they did not. These 
figures, although in every respect analogous to those of the seal, 
were lighter than the surrounding parts when first exposed to 
light, then they became darker, and remained so for several days. 
This may suffice to prove the complexity of these appearances ; 
the attempt to unravel the mysteries cost me considerable pains, 
by which other facts were brought to light, which increase still 
more the strangeness of the condensation of vapours. ‘The va- 
pours of mercury seemed to me to be best adapted for the ex- 
amination of this subject; they act with great delicacy, and may 
be employed at any degree of tension. It was not exactly neces- 
sary to examine images produced by contact; the common ones 
of Daguerre do not differ from them in any particular, and con- 
sequently they must lead to the same result. Iodized plates of 
silver, frequently those that had been exposed to chloride of 
iodine, were allowed to remain the proper time in a camera ob- 
