Tests for Potassium Salts

Platinic chloride added to solutions containing potassium salts gives a yellow, crystalline precipitate.

Potassium Sulphate


White, hard crystals. Soluble in 10 parts cold, and 4 parts boiling, water. Insoluble in alcohol. Used in some developing formulae in place of sodium sulphate.

Potassium Sulphide


Leather-brown or yellowish-green pieces. Deliquescent in moist air. Soluble in 2 parts of water, with but a slight residue. Gives an alkaline yellowish-green solution. Used to precipitate black silver sulphide from old fixing baths.

Potassium Sulphocyanide

KSCN. Potassium Sulphocyanate. Colorless, prismatic crystals. Deliquescent in the air. Easily soluble in water and in alcohol. Used in various toning processes. Sometimes employed as a developer in the carbon process, owing to its power of dissolving gelatin. EXTREMELY POISONOUS.


A precipitate is solid matter thrown down from a state of solution by the action of heat, light or chemical reagent.


Usually a written statement of the medicines or remedies to be used by the patient and the manner of using them.


A chemical used to preserve the keeping qualities of sensitized papers and plates, also solutions. Ex. Sodium sulphite or potassium metabisulphite are used in pyro developer to preserve the pyro, i. e., to keep the developer from oxidizing too rapidly during the process of development, and to preserve the solution before use.

Principal Axis

(See Axis, Principal.)

Principal Focus

VI. (See Focus, Principal.)


A positive image obtained from a negative on paper, either directly or indirectly.

Print Trimmer

A knife or instrument of any kind employed for trimming prints.


The term applied to the method of obtaining positive pictures from the negative, on paper, or any other suitable material.

Contact Printing

A printing term which signifies that the paper is placed in contact with the negative when making the exposure.

Depth of Printing

The extent to which printing must be carried in order that the resultant print may be of the proper strength.

Dodging in Printing

(See Dodging in Printing.)

Printing Frame

A frame in which is placed the negative, and on top of this the sensitive surface of the printing paper, which latter is pressed into close contact with the negative, when it is ready for exposure to light.


A term designating the addition of clouds, figures, etc., to a print from another negative, and which has spaces specially masked out for their insertion.


A term applied to any printing process in which the image becomes visible during the process of printing, being entirely produced by the action of light.