BLEACHING. Light has a very powerful effect in bleaching linens, woollen, and other fabrics from which the colour is capable of being discharged, though its effects in this way have not received the attention which they demand. The action of light has often been described as a deoxidizing action, but this is a very loose and incorrect statement of facts. There is no doubt that its effects on some bodies are thus correctly described, but it is equally certain that it exerts on other bodies the powers of an oxidizer; the oxygen which it removes from one class of substances it causes to combine with another class. Its effects, for instance, on organic compounds, are many of them deoxidizing, but are of an opposite character on most organic matters. Nascent oxygen is found in chemical opera tions to have a powerful bleaching influence, and it is probable that in those cases where light is applied as a bleaching agent, its specific action is to ozonify the oxygen of the air in contact with the coloured material, and thereby cause it to unite with and destroy the dye. Chlorine and some of its compounds are strong bleachers, but it is found that they are nearly or quite inert if no moisture is present : the chlorine decomposes the water, uniting with its hydro gen to form hydrochloric acid, and liberating oxygen, which is the real bleaching element. Now, the decomposition of water by chlo rine in the sun's light lies at the foundation of photography, and hence the superior efficacy of chlorine and light together over chlo rine alone. In many cases the chlorine itself is combined with organic matter by the sun's power, just as oxygen is ; and it should be remembered that chlorine, bromine, iodine, and oxygen are, as to their electro-chemical and actino-chemical properties, bodies of the same class. If light is a deoxiclizer, it is also a deodizer and de chloridizer, and if under certain conditions light becomes an oxidizer, it may also in analogous cases become an ioclizer. Thus, in photo graphy we not only reduce iodide of silver, we also, by changing the conditions, form it. Amongst " Miscellaneous Processes " will be found some where reduced and blackened silver salts are again compounded with chlorine, 8tc., and bleached. That effect, which has been called the reverse action of light, where the usual positive and negative effects are reversed, is probably an analogous chemical change. The substances used in the arts in bleaching photographic
and other papers, and otherwise preparing them for the market, are often injurious ; this fact should be attended to, and at the same time photographers will do a kindness to bleachers, dyers, and others, if they ean make their art practically influential in the arts in general.
BLIS'rERING. Blistering of the film is a defect which sometimes occurs in albumen processes on glass. It happens on spots where the albumen does not adhere tightly to the glass. The albumen film, like every other film which is capable of imbibing moisture, expands on being wetted. Now should any part of the film be non-adherent to the glass while the surrounding part adheres tightly, the non-adher ing part will expand on being wetted, and rais' e itself into a blister. The proper remedy for this defect appears to be, first, to clean. the glass with ammonia or soda instead of alcohol or nitric acid; and secondly, to add a little ammonia to the albumen. The reason of this will be evident from the following considerations. Albumen is a sticky substance, and will adhere to glass like any other sticky substance, provided its surface next the glass be not coagulated. Should, therefore, any trace of nitric acid or alcohol remain on the glass, it would be likely to coagulate the albumen, and render the film non-adherent in those parts, while the presence of an alkali, either in the glass or the albumen would tend to prevent coagula tion, and the albumen would adhere to the glass when dry, in the same way as any other sticky varnish. In all the albumen proce-sses on glass, the nitrate bath is strongly acid, and therefore an alkali may be added to the albumen with perfect safety.
The cases of blisteting which occur in the albumenized collodion process on glass may be traced to the same cause. The film of collodion is not sufficiently adherent to the glass in those parts where blistering occurs, to prevent the swelling of the albumen from forming a blister. In this process the collodion film should be as porous and adhesive as is consistent with the fulfilment of other conditions ; the nitrate bath in which the collodion is excited should be either neutral or slightly alkaline, and the excess of nitrate of silver should be thoroughly washed out of the collodion film.
BLorrniG PAPER. See " Filtering Paper."