A.—PREPARATION OF TAPER WITH SALTS OF SILVER. lb

pended in some viscid fluid, or by alternately washing with the nitrateof silver and some hydriodic salino solution.

By these means papers may be prepared which are exquisitely sensi-tive to luminous influence, provided the right proportions are hit; but,at the same time, nothing can be more insensible to the same agency,than the iodide of silver in some forms of combination. Theso singulardifferences in precipitates to all appearance the same, led to the beliefthat more than ono definite compound of theso elements existed. Ex-periment has, however, proved that the blackening of one variety of iodi-dated paper, and the preservation of another, depends not on two definitecombinations, but on the simplo admixture of a very minuto excess ofthe nitrate of silver. The papers prepared with the iodide of silver liavoall the peculiarities of those prepared with the chloride, and although,in some instances, they seem to exhibit a much higher order of sensi-tiveness, they cannot ho recommended for general purposes, with thatconfidence which experience has given to tho chloride. It may, how-ever, be proper to state tho best proportions in which the iodidatedpapers can bo prepared, and the best method of applying the solutions.

The finest kind of paper being chosen, it should be pinned by its fourcorners to a board, and carefully washed over with a solution of sixgrains of tho nitrate of silver to half an ounce of water; when this isdry, it is to be washed with iodide of potassium, fivo grains in tho samequantity of water, and dried by, but at somo little distance from, thofire; then, somo short period before the paper is required for use, itmust bo again washed with tho silvor solution, and quickly dried, withthe samo precaution as before. If this paper is warmed too much indrying, it changes from its delicate primrose colour to a bright pink ora rosy brown, which, although still sensitive, is not so much so as thoparts which arc not so altered. The peculiar property of this salt tochange thus readily by calorific influence, and some-other very remark-able effects produced on already darkened paper, when washed with ahydriodic salt, and exposed to artificial heat, or the puro calorific raysof the spectrum, which will bo horoafter noticed, appears to promiso aprocess of drawing which may be called thermography. Opening asthis does a wide range of highly interesting and most important experi-ments, it is to be hoped somo ono may pursuo tho subject, and endea-vour to establish the peculiar phenomena which present themselves, onsome scientific basis.

d. Bromidated Paters.

In many of tho Works on chemistry, it is stated that the chloride istho most sensitive to light of all the salts of silver; and when they areexposed in a perfectly formed and puro stato to solar influence, it willbe found that this is nearly correct. Modern discovery has, however,shown that these salts may exist in peculiar conditions, in which theaffinities arc so delicately balanced, as to be disturbed by tho faintestgleam; and it i^ingular, that as it regards the chloride, iodide, and