With certain kinds of developers there will form, after a lapse of twenty, fifty, or sometimes of sixty or eighty seconds, on the surface of the liquid floating figures of a description without doubt is slightly acid, the chances are not one in a hundred that the trouble is there. So, in such a case, examine the collodion; if it is a very pale color, it is neutral and perhaps alkaline, This is the very worst state it could be in if clear negatives are desired. An alkaline collodion coming contact with an acid hath causes a slight effervescence, which is fatal to a perfect union between the collodion and bath. To secure perfect harmony between them, if one is slightly add the other should also be. Such a condition is far preferable to a neutral one.

161. The nitrate bath probably comes in for more censure than any of the solutions used in photography. It is often innocent, and more often exactly similar to the appearances produced by this sort of fogging upon the plate. They consisted of groupings of particles of metallic silver, changing their shape and figure at very instant, but always having a resemblance to the figuring of marbled paper, which is made, as every one knows, precisely in a similar way, by the distribution of colored films on the surface of water, from which they are transferred to the surface of the paper. Just in the same way these curious silver figures arc presently transferred to the surface of the plate, to the inexpressible annoyance and discomfiture of the operator. Why films thus loosely deposited should adhere with such tenacity to the collodion, it is difficult to say; perhaps they are presently soldered fast by the advancing deposits of silver.

Almost all developers when examined in this way, by daylight in a capsule, showed a tendency to this form of fogging if their action was continued long enough. Developers which quickly become muddy show these figures simultaneously with the muddiness, but they were smaller and less marked. Strong and well-balanced developers did not show them nearly so soon; but they were larger and most conspicuous, and would present themselves when there was not the slightest turgidity in the liguid. It seems like a sort of reversed development - a development upwards instead of downwards.

These spots first form and show themselves on the surface of the liquid, on which they float freely. "When the plate is tilted in order to pour off the developer, they do not follow the liquid; but the latter slips out from between, and deposits them delicately on the film. It would seem, then, that if the plate were carefully watched, keeping the eye on the surface of the film, as well as on the developed image, and the moment a tendency to the formation of these figures was detected, the operator, instead of pouring off the developer, was to quickly set the plate (always holding it quite level) under a good stream of water issuing in a rose, these figures could not get down to the film. It is not their nature to sink through; they seem to adhere to the surface, and only reach the film by being left behind by the retiring wave of water, and then the whole stratum of developer is suddenly washed away by a stream of water which floods the whole surface. The floating figures must be carried away without an opportunity of fastening themselves to the film. - M. Carey Lea.

161. I could write a chapter expressive of my annoyance at that disgrace to professional photography - a dirty operator. In no respect does he more exhibit his obnoxious character than in the way he misuses the nitrate of silver solution. He splashes the walls with it, stains the So corrodes the metal - work, rots the dark-slide and camera, spoils curtains, chairs, and carpets, saturates his clothes, dyes his hair, blackens his hands, and defaces his abused;and misused. While it is the photographer's best friend, it is hated by him as though a "poor relation," and for years the inventive genii of the craft have been racking their brains for some means of abolishing it. The bromo - gelatin process promises much, but we cannot give up the nitrate bath for a time yet. Therefore, a few more thoughts concerning it and its treatment properly belong here. If difficulties occur with it, nine cases out of ten they result from the neglect of yourself. It may be you abuse it - it may be you have overworked it. To discover the trouble, the first step is to test its strength by any of the well-known methods. Pile's silver-test is the best.

162. Oftentimes a simple means of rectifying the bath is sufficient to put it in perfect working order. If it is too acid, cyanide of potassium countenance. The idea never seems to occur to him that the solution was not intended for any one of these purposes, and that it is not only wasted, hut doubly so, by doing nothing but injury by its misuse. - Alfred Hughes.

This method, which has been proposed by Professor Towler, consists, in brief, in precipitating the metal by means of zinc, drying and fusing it to a metallic globule, and then weighing it. Take an ounce of the bath solution. Next, in a porcelain vessel place a slip of zinc weighing about half an ounce, together with an ounce of water and a drachm of sulphuric acid. Effervescence will at once take place, and during this the silver solution is stirred in. The silver rapidly deposits as a gray powder. After a few minutes, brush off the silver from the zinc with a hair pencil, add water, and wash several times, taking care that no silver is allowed to escape. Now drain off the water and dry the mass; mix it with about twice its weight of borax, and fuse it by a blow-pipe flame. The quantity of silver per ounce of solution will thus be seen. But Professor Towler's method, as described, is not quite so good as the following method: Take a measured ounce of the bath solution and immerse in it a strip of magnesium ribbon, stirring well. Silver is at once thrown down, and after ten minutes a few drops of acetic acid are added to dissolve all traces of magnesium. The silver deposited is free from all impurities. It is washed and fused as before. Twelve grains of magnesium will precipitate one hundred and eight grains of metallic silver. This, as I have said, is a more convenient, simple, and accurate way of testing the strength of a silver bath than that proposed by Dr. Towler, and it can be very strongly recommended. - J. Traill Taylor.