LEADEN PIPES, CISTERNS, &c. When pure rain water is col lected in leaden tanks, or passed through leaden pipes, the lead is oxidized by the air contained in the water, and a small quantity of the oxide formed is dissolved in the water, thereby communicating to it poisonous qualities. The same thing does not, however, happen to the same extent when river or spring waters, which contain in general certain soluble salts, are retained in leaden vessels, because then the surface of the lead soon becomes coated with an insoluble precipi tate of sulphate, or carbonate, or phosphate of lead, which protects it from being further acted on by oxygen.
The presence of oxide of lead in the water used for photographic purposes may in some cases be rather beneficial than otherwise. For instance, in the pyrogallic developer, when freshly made, it would increase the energy of the development ; but the developer would not keep ; in a few hours, perhaps, it would become discoloured. Similarly, oxide of lead in the nitrate bath would perhaps increase the sensitiveness of the c,ollodion ffim, but render the bath liable to " fog " the picture.
Lead may be easily detected in water by tests which the photo grapher generally has at hand. Iodide of potassium would give a yellow tinge, sulphuric acid a white cloudiness, sulphide of ammo nium a dark discoloration to water containing oxide of lead in quantity sufficient to render its presence sensibly injurious, or other wise.
Any brass work in contact with lead would set up electric action, and assist the production of oxide of lead.
Bottles should not be made of flint glass, as some chemicals act on the lead which it contains. There is no lead in crown glass, and this should always be used for bottles intended to contain chemicals.
Achromatic lenses should never be exposed to the fumes which sometimes prevail in the laboratory, or dark room. The surface of the flint lens is frequently injured from this cause, and becomes iridescent.