The preservative Oxymel must be carefully filtered, and kept _covered_, in order to protect it from dust. It will also be necessary occasionally, before using it, to run it through a piece of white cambric, to stop back suspended particles, which, if allowed to remain, would be a source of spots. If it becomes mouldy, or discoloured by Silver, or ferments and evolves gas, throw it away. After the syrup is applied and the plates are drained, stow them in a grooved box perfectly protected from light; or place them in slides, which must be kept scrupulously clean, since any trace of impurity would be likely to produce a stain when the plate was left a long time in the slide. If the preserved plates are kept in a cupboard or box, see that no volatile matter, such as Ammonia, coal-gas, etc., can find entrance. In changing the plates after the exposure in the Camera, use a large bag made of _several thicknesses_ of black calico, with a square of yellow calico let in at the top; an elastic band securing it round the waist. THE COLLODIO-ALBUMEN PROCESS. This process, the theory of which has been briefly explained at page 181, is more sensitive than the one last described, and has the additional advantage of giving _dry_ plates, which do not attract dust, and are less liable to injury. The details of manipulation are complex, but this inconvenience is not so much felt when preparing a large number of plates. _Cleaning the Glasses._--Success will greatly depend upon the mode in which this part of the process is performed. The layer of Albumen which is applied to the Collodion film tends to swell and to raise the latter in blisters; the most effectual mode of obviating which will be to clean the glass so that the film adheres with unusual tenacity. The Liquor Potassæ of the Druggists, diluted with three or four parts of water, and rubbed on the glass by a roll of flannel (page 214), is very effectual. A mixture of Tripoli-water and Nitric Acid may however, if desired, be substituted:-- Tripoli 1 drachm. Nitric Acid 30 minims. Water 1 ounce. Lay the glass flat on a cloth, and rub the surface carefully with a tuft of cotton-wool dipped in the Tripoli; then, before the cream dries, wipe it off with a second tuft, and polish with a third. Lastly, breathe upon the glass, and having ascertained that it is chemically clean, apply the Collodion. _Coating with Collodion._--Choose a rather thin Collodion which adheres tightly to the glass. A preparation which has been kept a long time after iodizing will usually answer the purpose very well, and, as a rule, a non-contractile, structureless Collodion is better than one which is glutinous and wavy. The degree of sensibility of the Collodion is not thought to have much influence upon the result.