2. _Diffused Light._--_a._ In the developing room. This is a frequent cause of fogging, and especially so when the common yellow calico is employed, which is apt to fade. Use a treble thickness, or procure the waterproof material, in which the pores are stopped with gutta-percha.--_b._ In the Camera. The slide may not fit accurately, or the door does not shut close. Throw a black cloth over the Camera during the exposure of the plate.--_c._ From direct rays of the sun or the light of the sky falling upon the lens. With the full aperture of a double combination Lens, a portion of sky included in the field (as for instance to form the background of a portrait) is apt to cause fogging. The portrait will probably be more brilliant if a funnel-shaped canvas bag, or a curtain with an oblong aperture admitting only the rays proceeding from the sitter, be placed in front of the Camera. 3. _Alkalinity of the Bath._--This condition, explained at page 88, may be due to one of the following causes:--_a._ The use of Nitrate of Silver which has been too strongly fused (p. 13).--_b._ Constant employment of a Collodion containing free Ammonia or Carbonate of Ammonia (p. 89).--_c._ Addition of Potash, Ammonia, or Carbonate of Soda to the Nitrate Bath, in order to remove free Nitric Acid (p. 89).--_d._ Use of rain-water or hard water for making the Nitrate Bath (rain-water usually contains traces of Ammonia; hard water often abounds with Carbonate of Lime). In either case the alkalinity may easily be removed by the addition of Acetic Acid, one drop to four ounces of the solution. The proper mode of testing for alkalinity is described at p. 89. 4. _Decomposition of the Nitrate Bath._--_a._ By constant exposure to light (the injurious effects of this will be mostly seen when Positives are taken).--_b._ By organic matter: this is sometimes present in Nitrate of Silver which has been prepared from the residues of old Baths; or it may be introduced by floating papers for the printing process upon the Bath, or by dissolving the crystals of Nitrate of Silver in putrid rain-water, or in impure distilled water collected from the condensed water of steam-boilers and contaminated with oily matter.--_c._ Decomposition of the Bath by contact with metallic iron or copper, or with a fixing agent, or a developing agent (p. 90). 5. _Faults of the developing solution._--a. Brown and decomposed solution of Pyrogallic Acid; this may sometimes be used with impunity, but it tends, as a rule, to facilitate irregular reduction of Silver.--h. Impure Acetic Acid having a smell of Garlic and which probably contains Sulphur in organic combination.--c. Omission of the Acetic Acid in the developer: this will produce a universal blackness.