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3, 1862' Feb. 28, 1862.] THE PHOTOGRAPHIC NEWS. 99 ivalent; is founded on the fact that nitric acid in a strongly alkaline When zinc is used only about half the nitrogen is evolved as care to produce a perfect picture, so long as we employ wood chloride of iron prepared from a known weight of metallic iron. An excellent way of detecting the presence of small quantities of nitric acid when the exact amount is not re- n excess it dissol! to its ri fine wh A striking illustration of the powerful action possessed by an almost imperceptibly small portion of matter has lately come under our notice. An experimenter has been occupied for some time upon certain compounds which the element tellurium forms with organic radicles. Amongst others, he prepared some tellurium-methyl. The most re markable property of the tellurium-methyl compounds is their intolerable and persistent odour. A small quantity of the S 4 ’ his sec almost 4 I or evd entrusted to it during the exposure, and it returns it to the laboratory pure and uncontaminated by this contact, or in fected with the germ of stains, spots, rents, and other abominations, accordingly as itself is free or not from im perfection. Slides for wet collodion plates are, I believe, all made pre cisely alike, the model dating from the discovery of the process. There is no reason to desire any alteration in the main features, but I think a little attention may be use fully paid to some points of detail. I mentioned, when speaking of camera bodies, that when, for the purpose of introducing a diaphragm, the camera was made larger than the picture, it did not follow of necessity that the plate-holder should also be larger in the same pro portion. There is advantage to be gained, however, in having it made so, as the prepared plate then, instead of resting upon the corners of the plate-holder itself, may be held in an inner chassis, moveable, and more easily cleaned after every plate. Since the cleanliness and consequent worth of the picture depend much upon the plate not being soiled in the slide, no care should be neglected to guard against the possibility of impurity inside this latter, and whatever facilitates the cleaning of it, or that tends to pre- vnt the deposit of hurtful matter in it, increases in the same measure our chances of obtaining a perfect picture. In the earliest cameras the corners of the plate-holders and of the chassis were fitted with pieces of glass to support the plate, Which practically served as a resorvoir for a puddle of nitrate of silver, that, draining off the plate settled in these corners between the glass and the wood of the slide or chassis. This nitrate of silver became at once contami nated with organic matter, and if any of it came again in contact with the iodized surface of the plate it was a certain source of stains. When it is considered that the plate and holder had to be carried in this condition from the labora tory to the glass-room and back, and that the plate had to be taken out and its position reversed before development, all which encouraged the impurities to flow over it, it will be understood how difficult it was with such an arrange ment to get a plate free from all stain, and, in fact, how rare, in those days, it was to get a plate without at least a mark at the corners and edges, and how often these misfor tunes used to obtrude themselves in more important parts of the picture. This has been remedied to a great extent by substituting silver wires instead of the pieces of glass for the plate to rest upon. By this alteration the formation of a pool at each of the corners is avoided, but it does not entirely get rid of the chance of organic matter being carried on to the plate. The pools of nitrate of silver that have been noticed were only objectionable because they were formed in contact with the wooden frame, and became contaminated with organic matter, and in this state would stain the plate. The present chassis fitted with wires, is also made of wood, and when the plate holder is in a vertical position the plate rests upon this wood, and the point of contact receives the drain ing of nitrate, which—after imbibing impurities from the wood—when the slide is laid horizontally, or when the plate is removed, or even by capillary action only, may flow over the plate and stain it. It is usual to lessen this danger by hollowing out the sides of the chassis, so that the plate can only touch any side at the two extremities. This leaves eight channels by which impurities may be carried on to the plate at one stage of the process alone, and I think many otherwise inexplicable misfortunes might be traced to this source. I do not think we can be said to have taken every possible readily 1 a prod* possible,, -uun uuuu auu puloiovene ouvui. a ouan quauuvy JI Id there either the body itself or one of its salts allowed to get on to It will substav” many P. , with P0 quired to be ascertained, has also been given by’ Schulze. It of alcov is founded on the fact that nitric acid in a strongly alkaline > by dis/ solution by the action of finely divided zinc or sodium, or nufacte? aluminium amalgam becomes transformed into ammonia, ecipitati s tmIa - . Jioiaer.— mne plate-nonaer, or wet snae, is rne " • . I * - " . ° .* 7 1 ymenfAmost important part of the whole camera. On its condi- for any part of our apparatus that is to come into actual Asoluhjtion and nature ‘depends materially the condition of the contact .with a wet .collodion plate, or in such very close nts brisH photographic impression. The plate-holder alone comes in proximity to itthat the silver drainings from the plate can t the factual contact with the delicate sensitive plate, which is touch the wood, and by any chance get returned to the rvation J l plate.. ent of • * Continued from page 89. | It is usual to varnish the inside of the slide, and the e odour. , , Cameras.—Continued. 3 this P The. Holder.—The plate-holder, or wet slide, is the Lmnct mno. A.. 3 arts. ' rong s0 id w otf0l ammonia, but with sodium amalgam the change is complete, acig o When a solution is to be examined it is to be supersaturated id matt vith caustic potash and boiled for some time until the allow of’ original ammonia is got rid of: the sodium amalgam is are m. th en dropped into the residual liquid and the mixture again otograpb "Sated. If the smallest traces of nitric acid are present in latenr clessolution it will be rendered evident by the immediate smelition of ammonia, which may be detected by the y mentj i has j, * recipita --MARKS UPON SOME OF THE APPARATUS arines0 employed in photography* taric ac I of frei BY F. R. WINDOW. the finger, soon communicatas a smell to the whole body, low tod and in a few days is perceptible in the breath. The stench delique is so lasting that the unfortunate chemist is shut out from insider^ society for several months. We mention this because there ad has disa tendency with some experimentalists to introduce the compot salts of th eS c compound ammonias into photography, and if ions wbi the fashion be once set, there is no knowing where it would 1 purpo stoP. It > s not likely that tellurium would be at present perly o introduced into photography, owing to its expense ; but this esmay)not being due to the rarity of the metal as much as to its ; process want of utilization, may at any moment cease to be a bar to ost pur its employment. ie prep 411 It is often of consequence to the photographic chemist to perimed ascertain the exact quantity of nitric acid present in a solu- of a crd‘ tion. The ordinary processes for estimating this are tedious >er cent, in the extreme, and, consequently, recourse is generally had e added’ to the volumetric process of analysis proposed by Pelouze. re filter This has lately been modified by Braun in the following hours.pay:—Instead of estimating by permanganate of potash 5 get, fethe amount of protoxide of iron on which the nitric acid •sit wbhas not acted, he determines the peroxide of iron formed by ttle of 1 ? moans of iodide of potassium and hyposulphite of soda, of alco Protochloride of iron is first formed, which is then per own do oxidised to its fullest extent by the nitric acid whose minou amount it is wished to ascertain. Iodide of potassium is re or Ithen added, when the perchloride of iron, as is well known, i alcob sets the iodine free, even in the cold, but more completely e maj ] when the reaction is assisted by a gentle heat. The amount as long of free iodine present may now be easily estimated by means The 19 of a solution of hyposulphite of soda after having added a red inf small quantity of starch-paste. The standard solution of J hyposulphite of soda may be prepared either by dissolving a known weight of the salt in a similarly known "weight of water or by standardizing with a solution of hyposulphite of unknown strength the iodine set at liberty by per-
