78 



" Luppo-Cramer, (Theodor Steinkopf), Dresden (1008). ' Koloid-Chemie 

 und Pliotographie.' 



Several papers are publislied each year on this subject by Xiuppo- Cramer 



in the Kolloid Zeitschrift. 



1' Sheppard & Mees, Zeit. f. wiss. Phot., 1, 27 (1909). ' Tlieorie der Photo- 

 grapliischen Prozesse : Reifen und der Photoelcktrisclie Effekt.' 



"A. P. H. Trivelli, Zeit. f. wiss. Phot, 8, 17 (1910). ' Beitrag zu einer 

 Tlieorie des Reifungsprozesses der Silberhaloide.' 



19 Sichling, Zeit. Phys. Chem., 77, 1 (1911). ' Uber die Natvu- der Photo- 

 chloride des Silbers und deren Lichtpotentiale.' 



2» W. D. Bancroft, Jour. Phys. Chem., 15, 313, 551 (1911); 16, 27, 89 (1912). 

 ' The Photographic Plate ' : The Latent Image, Pt. I., II., III., IV. 

 A summary of the literature up to that date. 



21 Allen, The Photographic Journal, 54, 175. ' The Formation of the Latent 

 Image.' 



2- Lorenz und Eitel, Zeit. Anorg. Chem., 91, 57 (1915). 'Uber Silbemebel 

 in Silberchlorid und Silberbromid.' 



' 23 Lorenz imd Hiege, Zeit. Anorg. Chem., 92, 27 (1915). 'Uber den 

 Belichtungsvorgang in festen Silberchlorid und Silberbromid.' 



" Krohn, Phot. Jour., 58, 179 (1918). ' The Mechanism of Development of 

 the Image in a Dry Plate Negative.' 



25 Ritz, Oeuvres de Ritz, 4, 80. 



2 6 Abney, Phil. Trans., 171, II., 653 (1880). 'The Photographic Method 

 of Mapping the least Refrangible end of the Solar Spectrum.' 



2' Ritz, Comptes Rendus, 143, 167 (1903). ' Sur la Photographic des Rayons- 

 infrarouge.' 



References to the papers on optical properties of colloidal solutions 

 are given under the next section. 



Colloidal Silver and the Colour of Silver Deposits. 



Colloidal solutions of silver in the finest state of division are 

 yellowish brown in colour and as the size of the particles is increased 

 the colour changes to ruby red, Ulac, and blue. When these solu- 

 tions are precipitated by the addition of an electrolyte a dark 

 grey precipitate of sUver is obtained. If a soluble sUver salt is reduced 

 by a powerful reducing agent such as alkaline pyrogallic acid, silver 

 is precipitated in the black form, if a less powerful reducer such as 

 pyrogaUic acid alone, is used, a grey precipitate is obtained^. 



Any circumstance which tends to prevent the coalescence of 

 the reduced silver such as the reduction of an insoluble salt, or the 

 enclosure of the salt in gelatin, yields the dark modification. Collodion 

 does not hinder the coalescence of the silver particles to nearly the 

 same extent as gelatin^. 



The colour of the image obtained varies to some extent with the 

 developer used. If gaslight paper is developed with an excess of 

 bromide in the developer the first image becomes red, but appears 

 black as the develpoment proceeds. The products of oxidation of 

 some developers stain the gelatin yeUow or brown as these oxidation 

 products are present in the greatest concentration at those parts of 

 the plate which have been most exposed, the silver deposit in the 

 negative appears to be stained through, though this is not the case. 



1 Luppo-Cramer, Zeit. f. Chemie und Ind. der Kolloide,^ 3, 33, 135, 170 (1908). 

 ' Uber das Silbergel in den photographischen Schichten.' 



