[Concluded.]
With the view of testing Dr. Draper's results, I carefully iodized two silver plates and exposed them to light. I then placed them so that half of one plate was covered by half of the other, and allowed them to remain in the dark 1-24th of an inch apart for four hours. On mercurialization I could not detect the slightest difference between the covered and uncovered portions of either of the plates.
Another silver plate was iodized and exposed to light. It was then placed in the dark wish a sensitive plate which had been carefully kept from the light, 1-16th of an inch above it, and a small engraving placed between them. They were allowed to remain thus for six hours. When exposed to the vapor of mercury, the plate which had been subjected to the light whitened all over, and the space occupied by the engraving was distinctly marked by lines of vapor thicker than the other parts. The plate which had been preserved in the dark was scarcely at all influenced by the vapor, except on those parts which had been touched by the supports of card-board on which it rested. These were so arranged that no radiation could have influenced those parts of the plates.
An iodized silver plate was placed in the dark with a little fine string coiled over parts of it, and a polished silver plate supported 1-8th of an inch above it. After four hours both plates were subjected to mercurial vapor. On the iodized plate the deposit of vapor was uniform, although slight; but on the superposed plate of silver a strong and beautiful image of the string on the under plate became visible. I found that neither of the two iodized plates had lost their sensitiveness by the operations to which they had been subjected in the dark.
Hoping to detect some evidence of the process by which these singular results were produced, I instituted a series of experiments, of which the following are some of the most interesting results.
A. A silver plate was iodized, a piece of card was placed upon it, and a well-polished mercurial plate (amalgamated copper) was suspended 1-8th of an inch above it, and left in this state for a night. The space on the silver plate corresponding with the mercurial plate, except under the card, was nearly freed of its iodine, which had evidently combined with the mercury on the upper plate. On exposing the mercurial plate to the vapor of mercury the image of the card was rendered visible, the vapor covering every part of the plate except that opposite the card. The silver plate received the vapor only on those parts which were not influenced by the mercurial plate. The upper plate was suspended by strings; these were faithfully imaged on both plates; by a thick line of mercurial vapor on the under plate, by the absence of it in the upper one.
B. An iodized silvered plate was exposed to light until brown, and a mercurial plate suspended above it for twelve hours. The browned silver plate was whitened, and all the irregularities of the mercurial plate strikingly marked on it: the mercurial plate was slightly tarnished. On rubbing the silvered plate it was found that the silver was removed more readily over the whitened portion, but had lost none of its adhesion in other parts.
C. Over an iodized silver plate, plates of gold, platina, silver, brass, copper, copper amalgamated, and zinc were placed at the distance of 1-8th of an inch. After three hours the amalgamated plate had made a decidedly visible impression on the silver one. On exposure to vapor, the mercury lodged on every part of the plate except that affected by the mercurial plate; some irregularities were observed, but none which could be decidedly traced to the other metals in juxtaposition. I have some evidence that different metals near each other seriously interfere with each other's influence.
D. A mercurial plate was iodized, and another mercurial plate placed 1-8th of an inch above it. The upper plate became covered with a bright yellow film; and on exposing them to mercurial vapor, marks became apparent which corresponded with those in the opposite plate.
E. A silver plate was iodized and placed in the dark with an engraving, face down, upon it. An amalgamated copper plate was laid on this, and left for fifteen hours. The mercurial plate was reddened, and on exposure to the vapor of mercury, a very nice impression of the engraving was brought out, it having been effected through the thickness of the paper. On the silvered plate the space covered by the paper was well marked; but vaporization produced no trace of the engraving. The space beyond the paper was rendered white. It was curious that both plates had several spots which corresponded, particularly two, distinguished by a well-defined circle and a comet-like appendage, in length ten times the diameter of the circle. These spots could not be traced to anything visible in the print or either of the plates, and must, I think, be referred to some electrical influence. I find it indeed commonly the case, that the plates, after being subjected to these kind of experiments a few times, become mottled, or present on their polished faces all the appearances of a finely-grained wood, and in this state they are less susceptible of receiving any impression than when not so.