The patent plate answers better than any other description of glass; but if it cannot be procured, the "flatted crown glass" may be substituted. Before washing the glasses, each square should be roughened on the edges by means of a file or a sheet of emery-paper; or more simply, by drawing the edges of two plates across each other. If this precaution be omitted, the fingers are liable to injury, and the Collodion film may contract and separate from the sides. In cleaning glasses, it is not sufficient, as a rule, to wash them simply with water; other liquids are required to remove _grease_, if present. A cream of Tripoli powder and Spirits of Wine, with a little Ammonia added, is commonly employed. A tuft of cotton is dipped in this mixture, and the glasses are well rubbed with it for a few minutes. They are then rinsed in plain water and wiped dry with a cloth. The cloths used for cleaning glasses should be kept expressly for that purpose; they are best made of a material sold as fine "diaper," and very free from flocculi and loosely-adhering fibres. They are not to be washed _in soap and water_, but always in pure water or in water containing a little Carbonate of Soda. After wiping the glass carefully, complete the process by polishing with an old silk handkerchief, avoiding contact with the skin of the hand. Some object to _silk_, as tending to render the glass electrical, and so to attract particles of dust, but in practice no inconvenience will be experienced from this source. Before deciding that the glass is clean, hold it in an angular position and _breathe_ upon it. The importance of attending to this simple rule will be at once seen by referring to the remarks made at page 39. In the Honey preservative and Collodio-Albumen processes it is especially needful that the glasses should be thoroughly cleaned, on account of the tendency which the film has to become loosened or to blister during the development and washings. Caustic Potash, sold by the druggists under the name of "Liquor Potassæ," is very efficacious, or in place of it, a warm solution of "washing Soda" (Carbonate of Soda). Liquor Potassæ, being a caustic and alkaline liquid, softens the skin and dissolves it; it must therefore be diluted with about four parts of water and applied to the glass by means of a cylindrical roll of flannel. After wetting both sides thoroughly, allow the glass to stand for a time until several have been treated in the same way; then wash with water and rub dry in a cloth. The use of an alkaline solution is usually sufficient to clean the glass, but some plates are dotted on the surface with small white specks, not removable by Potash. These specks may consist of hard particles of _Carbonate of Lime_, and when such is the case they dissolve readily in a dilute acid,--Oil of Vitriol, with about four parts of water added, or dilute Nitric Acid.