6. _Sundry other causes of fogging._--_a._ Vapour of Ammonia or Hydrosulphate of Ammonia, or the products of the combustion of coal-gas, escaping into the developing room.--_b._ Development of the image by immersion in solution of Sulphate of Iron: this is a safe plan when the films are formed in an acid Nitrate Bath; but with pale films formed in a chemically neutral Bath it is better to pour the fluid over the plate, and not to use the same portion twice.--_c._ Redipping the plate in the Bath before development: this is apt to give a foggy picture when using an old Bath, and is not recommended. _Systematic plan of proceeding to detect the cause of the fogging._--If the amateur has had but little experience in the Collodion process, and is using Collodion of moderate sensitiveness and a new Bath, the probability is that the fogging is caused by over-exposure. Having obviated this, proceed to test the Bath; _if it is made from pure materials, and does not restore the blue colour of a piece of litmus-paper previously reddened by holding it over the mouth of a glacial Acetic Acid bottle_, it may be considered in working order. Next prepare a sensitive plate, and after draining it for two or three minutes in a dark place, pour on the developer: wash, fix, and bring out to the light; if any mistiness is perceptible, either the developing room is not sufficiently dark, or the Bath was prepared with a bad sample of Nitrate of Silver, or with impure Alcohol, or impure water. On the other hand, if the plate remains absolutely clear under these circumstances, _the cause of error may be in the Camera_;--therefore prepare another sensitive film, place it in the Camera, and proceed exactly as if taking a picture, with the exception of not removing the brass cap of the lens: allow to remain for two or three minutes, and then remove and develope as usual. If no indication of the cause of the fogging is obtained in either of these ways, there is every reason to suppose that it is due to diffused Light gaining entrance through the lens. This cause of error may often be detected by looking into the Camera from the front, when an irregular reflection will be seen upon the glass. SPOTS UPON COLLODION PLATES. Spots are of two kinds: spots of opacity, which appear black by transmitted light, and white by reflected light; and spots of transparency, the reverse of the others, being white when seen upon Negatives, and black on Positives. Opaque Spots are referable to an excess of development at the point where the spot is seen; they may be caused by-- 1. _The use of Collodion holding small particles in suspension._--Each particle becomes a centre of chemical action, and produces a speck, or a speck with a tail to it. The Collodion should be placed aside to settle for several hours, after which the upper portion may be poured off.