The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Positive                                              Potassium Carbonate
Positive. A reproduction of any object in which the lights and shades are represented as seen in nature, whether on glass or paper. A positive for viewing by transmitted light is some-times called a diapositive, and one for viewing by reflected light - like an ordinary print on paper - is sometimes distinguished by the term kata-positive. It is the opposite to negative.
Postage-Stamp Photographs, Camera for. See Multi-plying Camera.
Potassium Bichromate (Ger., Kaliumdichromat; Fr., Bichromate de potasse, Chromate rouge de potassium; Ital., Bichromatadipotassd). K2Cra07 = 295. Synonyms : Potassium Dichromate, Red Chromate of Potash, Acid Chromate of Potash. Is prepared on a large scale from chrome iron ore. It is met with commercially as fine orange-red crystals, which give a very deep-coloured solution. Solubility , 7*4 per cent, in cold, 9*4 per cent, in hot water ; insoluble in alcohol. It is of great importance commercially, the fact of its being decomposed by light when in contact with organic matter being taken advantage of. It is used for dyeing, and also for tanning hides, the action in this case being analogous to that in the carbon process. It is used, photo-graphically, for nearly every photo-mechanical printing process* It is also used in solution as a colour screen for orthochromatic work.
Potassium Bromide (Ger., Bromkahum, Kaliumbromid ; Fr., Bro?nure de potassium ; Ital., Bromuro dipotasio). KBr=ii9, Prepared in white cubical crystals by acting on bromide of iron with carbonate of potash, or by the action of the metalloid itself on caustic potash, and subsequent purification and crystallisa-tion. Solubility: 58 per cent, in cold, 102 per cent, in hot water; 0.13 per cent, in cold, 7 per cent, in boiling alcohol; o*2 per cent, in ether, 0.05 alcohol and ether; 25 per cent, in glycerine.
Potassium Carbonate (Ger., Kaliu?ncarbonat, Kohlensaures Kali, Potasche; Fr., Carbonate de potasse; Ital., Carbonato di potassd). K2C03,3H20 = 192. Synonyms: Pearlash, Subcarbon-ate of Potash, Salt of Tartar, Salt of Wormwood, Potash. It contains nearly always about 16 per cent, of water of crystallisa-tion, and is prepared by lixiviation^ and subsequent purification, of the ashes of wood and vegetable matters. Solubility: 100 in
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