174 DARK TITHONIC RAYS. 



768. I use the word " polarized" in its chemical sense. An illustration will serve 

 to show the signification I attach to the term. When water is placed between platina 

 electrodes, its oxygen is liberated from one of them, and its hydrogen from the other, 

 and the intervening liquid assumes a polar state, a series of decompositions and combi- 

 nations going on. As that water is polarized, and undergoes polar decomposition, so, 

 too, do the same phenomena hold in the case of the Daguerreotype film. 



1st. We know that no iodine is ever evolved from the plate, even under the most 

 prolonged action of the light (587). 



2d. The cause of the final appearance of the image is due to silver being liberated on 

 the anterior face of the plate (592). 



3d. When, by the action of gelatine, the iodine and mercury are both removed from 

 the plate, it is obvious that the plate has been corroded wherever the light fell. Iodine, 

 therefore, has been evolved on the posterior face of the film, and is the cause of this cor- 

 rosion. 



769. From the circumstance, therefore, that iodine is evolved at the back of the film 

 and silver at its front, and the film itself remaining the same in thickness throughout, it 

 is obvious that there is a strong resemblance between this phenomena and that of the 

 polar decomposition of water. The electro-positive and electro-negative elements are 

 yielded up on opposite faces of the film, and its interior undergoes incessant polar chan- 

 ges, the opposite electric particles sliding, as it were, on one another. 



770. In a note to the preceding chapter (708, &c.) I have described the remarkable 

 power of certain electro-negative gases in producing the rapid detithonization of sur- 

 faces that have been changed by light. Since that paper was sent to England, I per- 

 ceive, from the " Scientific Memoirs," that Professor MOSER has published results of a 

 similar kind. The true explanation of them appears to me to be very different from 

 that which he gives ; for his idea of vapours containing latent rays of particular orders of 

 refrangibility or colour, rests on a very feeble analogy, and strikes me as entirely without 

 support. 



771. The view which I have taken of these phenomena, and to which allusion was 

 made in the paper referred to, can be easily understood from what has just been said. 

 The film on a Daguerreotype plate, which has been disturbed by the tithonic rays, but 

 not yet mercurialized, is in a polar condition of force, its iodine is ready to unite with 

 a new layer of silver behind, its silver is ready to be evolved in front. If it be expo- 

 sed to mercurial vapours, union at once takes place on that front face, and an amalgam 

 is formed ; if to the vapours of iodine, or chlorine, or bromine, an iodide, chloride, or 

 bromide of silver is formed. In an instant, its disturbing affinities being satisfied, the 

 film reverts back to its former condition of equilibrium, and is precisely in the condition 

 it was in before exposure to the light. 



