Fogging. When a precipitate is thrown down over the entire plate by the action of the developer, so as to obscure, in the deepest shadows, the transparency of the glass when looked through, that precipitate is called "fog," and the picture is said to be "fogged."

The principal causes of fog are: alkalinity of the nitrate of silver bath, or neutrality of the fog. It is as insidious as foul air, and both causes and hides many other imperfections. It is caused in many ways. Among them the following surely produce fogging: 1, want of union between the collodion and the nitrate bath; 2, want of acid in the developer; 8, want of sufficient acid in the nitrate bath (or an excess of acid will sometimes pro-duee the same effect.4, diffused white light in the dark - room, camera, etc. caused by not shutting the door close; having cracks and chinks in the partitions; not having the yellow glass sufficiently dense to obstruct all the white light: taking the plate out of the hath too near the gas or lamp, and by developing too near the same: by want of care in redeveloping, and light reflected from surrounding objects, thereby obstructing the direct rays from the sitter, which alone should pass through the lens. It may also be caused by excessive over - exposure, and by underexposure with too long a development; also by keeping the plate too long after taking it out of the bath before development It is also very frequently caused by the names of ammonia, turpentine, and other volatile chemicals standing about upon the shelves of the dark-room. It is easy to produce foggy negatives without the chemicals being in fault.

159. How are we to tell, then, the cause of this monstrous trouble? We naturally look first at the chemicals. Is it in them or not 1 Bow same when a bromo - iodized collodion is used; over - acidity of the bath; diffused light, either in the camera or dark-room; dirty plates; and sometimes from the lens itself. An alkaline bath and a very acid one seem to have very similar properties as respects " fogging." It is only when the acid and alkaline elements are properly balanced, so as to suit the nature of the collodion, that a good photographic negative can be obtained. No collodion can be worked in an alkaline bath, but a pure simply iodised collodion when it turns to a pale sherry color, can be used with the best effect in an absolutely neutral nitrate solution. Collodion, paper, or any sensitive medium which contains besides the iodide an organic compound, must always be sensitized in an acid bath to prevent " fogging." Should the bath " fog " upon trying a plate, and the cause be in the bath, add five or ten drops of nitric acid, stir well, and try another plate. - Elbert Anderson.

159. "Fogging" is a general obliteration of the forms of the subject in an opaque film, which prevents them from being clearly distinguished, in whatever direction they may be wed. This is caused in a variety of ways. It may result from the unskilful use of the developer itself; if it is of too great strength in warm weather, when it should have been reduced in power by the addition of distilled water and acid, "fogging " will ensue, or, at a more moderate temperature, prolonging the time of development beyond a certain limit will cause the same blemish. If weakening the solution in the one case, and shortening the developer in other, does not remedy the evil, the nitrate bath must be tested for alkalinity with reddened litmus or turmeric paper.

When there is only a very slight tendency to "fog," it is better not to touch or alter the shall we determine it? We will make a search. Darken the room and light the gas. Take a perfectly clean plate, coat it with collodion, dip it as usual in the bath, and cover it over. While it is coating, take the plate-holder and lay it flat down, open it and put in a negative, varnished side up. Then lay a narrow strip of cardboard on each end of the negative, so that when the sensitized plate is put in it will be as near in contact with the other as possible, without - touching. The plate being now coated, turn down the light to the lowest point, remove the plate from the bath, and, after draining it carefully, lay it collodion side down upon the strips of cardboard, and fasten the frame. Now turn up the light, and hold the plate-holder three or four inches from the gas, draw the slide, and expose it four or five seconds; if the light should be a lamp, expose longer; shut the slide, and then turn down the light as before, and develop. The result will be a positive by transmitted light. If it is clear and brilliant, free from fog, there is no fault with the chemicals.

If on the other hand it proves foggy, the trouble is probably in the collodion, because, if the bath is made according to directions, and bath; by using a more highly colored sample of collodion perfect clearness of definition will be restored to the film, whilst at the same time every plate of such quality of collodion that is dipped will tend gradually to displace, more and more, the small tendency to alkalinity existing in the bath. Lake Price.

160. The developer will naturally reduce the silver on any part of the plate that has been "struck" by light; this will be in irregular masses, and only at such places of the plate as were " light struck." The collodion is rarely, if ever, the cause of fogging, for a collodion made as I have directed cannot be or become alkaline; for if it should be colorless vrhen first made, I have directed the use of a few drops of tincture of iodine, or the addition of one-fourth of its volume of an old and red collodion made by the same formula.

If the chemicals have been prepared according to the directions given, and the bath is slightly acid, as also will be the collodion, fogging cannot occur until the bath becomes disordered, or a change is made in the collodion or in the developer. Coat a plate carefully, and immerse it in the bath slowly; when properly coated, develop it (exactly as if it had been exposed, and keep on the developer for about the same length of time); wash off the developer under the tap and " fix." If a universal layer of mistiness cover the entire plate, and lie only on the film and not in it, the bath is in fault; if, however, there should be any portion of the plate perfectly clear, the chemicals are not to blame. Should the bath prove alkaline, which is not to be expected, the cause of fogging in this instance is the presence of organic matter, which a slight addition of nitric acid will dissolve. Should the bath prove acid, no more acid should be added at this, nor at any other time, to the bath. If a newly prepared bath " fog " but very slightly, let it stand quiet all night, and it will be found to work clean and bright on the morrow. - Elbert Anderson.