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Fig 127 M Machs Apiar Atus

glass, solution, washed and removed

FIG. 127 -M. MACH'S API'AR ATUS. a chemically clean piece of glass in as many hours.

Cleaning are many methods of cleaning a glass plate successfully. The application of Tripoli powder solution, rubbed well over the surfaces with a piece of old rag, is one to be recommended. This is then removed and a final polish given to the surface with a dry piece of chamois leather or an old silk handkerchief, which should be well washed in carbonate of soda and water, and well dried before use.

A brand of soap known as " Monkey soap," and used only in cleaning non-destructible articles, is an invaluable cleanser of glass plates, as it seems to entirely remove all grease and other foreign matters.

In polishing a glass plate positive electricity is generated on it, and negative on the rubber. This will often produce an evil effect upon the subsequent operations; for instance, if the glass is to be coated with collodion the electricity will prevent its proper adhesion, and it is therefore necessary to dispel the latter by passing the handkerchief gently and slowly over the surface to allow the two electricities to combine.

There are various methods of testing if glass be clean. The simplest is, perhaps, by breathing gently, and observing if the condensed breath leaves the plate regularly and evenly.

Glass which is exceptionally dirty should be immersed in a ten per cent. aqueous solution of nitric acid, then washed and rubbed over with a solution of caustic potash or soda half an ounce, and methylated spirit half an ounce, added to four or five ounces of water, or scoured well with the soap already recommended.

Greasy glasses or vessels must not be washed until the grease is removed with a dry cloth (only used for that purpose), and the glass or vessel washed in a ten per cent. solution of nitric acid. It is then washed and treated as described above.

Collodion can easily be removed from bottles by allowing the ether and alcohol to evapo rate, filling the bottle with cold water and using a bottle brush. Dry and rinse out with alcohol.

Albumen being soluble in alkaline solutions can be removed by applying a solution of caustic potash, and washing well with water. Old or spoilt gelatine negatives should be soaked in a solution of chromic acid, or in a warm solution of common soda. This will decompose the gelatine, and the films may be detached and the glasses washed. Another method is to soak in a weak bath of hydrochloric acid, and thoroughly wash and dry.

Old collodion films can be removed by immersion in Nitric acid i ounce Water to ounces Varnished the 'case of negatives which have been varnished, the varnish must first of all be removed by soaking the plate in strong spirits of benzine, and removing the soluble varnish with a piece of flannel or tuft of cotton wool.