380 Mr. A. Claudet on the principal Phcetiomena of 



enough for that purpose. In that state I submit the surface 

 acted on by a great number of intensities of light to the sub- 

 sequent radiation through red, orange, or yellow glasses, or 

 any other coloured transparent media, in order to examine 

 the action of these radiations on one-half of the effects pro- 

 duced by each intensity of light. By these means I have 

 found, that before light has decomposed the surface and pro- 

 duced the white precipitate, the red, orange, and yellow rays 

 destroy the affinity for mercury, and continue it when the de- 

 composition has begun. 



In the course of my experiments I noticed a curious 

 fact, which proved very puzzling to me, until I siicceded 

 in assigning a cause to it. I shall mention it here, because 

 it may lead to some further discoveries. I observed that 

 sometimes the spaces under the round holes, which had not 

 been affected by light during the operation of the photogra- 

 phometer in a sufficient degree to determine the deposit of 

 mercury, were, as was to be expected, quite black ; while 

 the spaces surrounding them were in an unaccountable man- 

 ner slightly affected by mercury. At first I could not explain 

 the phaenomenon, except by supposing that the whole plate 

 had been previously by accident slightly affected by light, and 

 that the exposure through the holes to another sort of light 

 had destroyed the former effect. I was naturally led to that 

 explanation, having before observed that one kind of light 

 destroys the effect of another; as, for example, that the effect 

 of the light from the north is destroyed by the light from the 

 south, when certain vapours existing in the latter portion of the 

 atmosphere impart a yellow tint to the light of the sun. But 

 after repeated experiments, taking great care to protect the 

 plate from the least exposure to light, and recollecting some 

 experiments of'M. Moser, I found that the affinity for mer- 

 cury had been imparted to the surface of the Daguerreotype 

 plate by the contact of the metallic plate having the round 

 holes, while the space under the hole had received no similar 

 action. But it must be observed that this phaenomenon does 

 not take place every time; some days it is frequent, and in 

 some others it does not manifest itself at all. Considering 

 that the plate furnished with round holes is of copper, and 

 that the Daguerreotype plate is of silver plated on copper, it 

 is probable that the deposit of mercury is due to an electric 

 or galvanic action determined by the contact of the two metals ; 

 and perhaps the circumstance that the action does not take 

 place every time, will lead to the supposition that it is developed 

 by some peculiar electric state of the ambient atmosphere ; and 

 by a degree of dampness in the air, which would increase the 



