Paraffin Wax

(See Paraffin.)

Sealing Wax

(See Sealing Wax.)

Apothecaries Weight

The weight by which formulae are usually, and most conveniently, made up.

Atomic Weight

Atomic weight is the relative weight of an atom of an element compared with that of hydrogen, which, being the lightest, is taken as the unit.

Avoirdupois Weight

The weight by which chemicals are usually sold.

Troy Weight

The weight usually employed for weighing precious metals.

Wet Collodion Process

An early photographic process. On account of its strong image and fineness of grain it is still largely employed for making halftone plates; also for copying, etc. The iodized collodion plate is sensitized in a silver nitrate bath and exposed while wet. The plate is developed in pyro, or an iron developer, and fixed in potassium cyanide.

Wet Plate Process

A process in which the plate is immersed in a sensitizing solution immediately before use (exposed while wet).

Whole Plate

A plate 6 1/2 x 8 1/2 inches in size

X-Ray Photography

- II. Rontgen Rays. The art of securing an image on a photographic plate of opaque bodies in various degrees, according to their density and relative thickness. The ray itself is invisible. Platinum, lead and silver will not allow the rays to pass through, and for this reason a lead plate is employed to protect the sensitive photographic plate from the X-rays. Aluminum, wood, paper, etc., do not obstruct the X-rays, neither are the rays refracted by prisms of carbon disulphide, while prisms made of vulcanized rubber, as well as those made of aluminum, give an uncertain refraction. The X-rays were discovered by Professor Rontgen, and were called X-rays on account of the difficulty experienced in classifying them. X-rays are produced by passing an electric current through a large vacuum tube, known as Crooke's tube. One of the greatest advantages of the X-rays is that it allows the physician to secure photographs through the tissues of the body, showing fractures of bones, or the presence of bullets and other foreign substances.

Yellow Screen

(See Color Filters.)

Zinc

Zn.

A metal of bluish-white color, having crystalline fractures. Soluble in acids. Use. Iron coated with thin layers of zinc (galvanized iron) is used for washing tanks, etc., as it does not rust. Zinc tanks and trays are also used to a considerable degree for this same reason. Sheets of zinc are employed in the half-tone process, for making line cuts, etc. The salts of zinc, however, are used but little in photography.

Zinc Chloride

ZnCl2.

White, granular, deliquescent powder. Soluble in about 0.5 parts water, 1 part alcohol, and ether at 6o° Fahr. Used principally as an antiseptic.

Zinc Oxide

ZnO

Zinc White or Chinese White. White powder. Absorbs carbon dioxide from air. Soluble in dilute acids, ammonia, ammonium carbonate; insoluble in water. Keep in air-tight vessel. Used in making paint.