79 



2 Rayloigh, Proc. Roy. Soc, 84, 25 (1910). 'The Tncidenco of Light upon a 

 Transparent Sjihere of Dimensions coinparal)le witli tlie Wave-lengtli.' 



'Chapman Jones, PJwt. Jour., 51, lo9 (1911); 57, 158 (1917). 'On the 

 relationship between tlie size of particles and the colour of the image.' 



" Nils l>liill)Mcl, Zi'i't. f. Chemie und Indu.strie der Koiloide, 9, 156 (1911). ' Zur 

 Kenntnis tier Lichtabsorption in Silborhydrosolen.' 



=> Swen Oden, Zeil. Phys. Chem., 78, 082 (1912). ' Beziehung zwischen 

 Teilchengrosse und Stabilitiit disperser Systeme.' 



« Keen and Porter, Proc. Roy. Soc, 89, 370 (1914). 'Diffraction of Light 

 by Particles comparable with the Wave-length.' 



' Paris, Phil. Mag., 30, 459 (1915). ' On the Polarisation of Light scattered 

 by spherical metal particles of Dimensions comparable with the Wave-length.' 



' (ians, Ann. d. Physik, 47, 270 (1915). ' Uber die P'orm ultramikroskopischer 

 TeQchen.' 



' Liesegang, R. E., Zeit. wiss. Phot., 14, 343 (1915). ' Uber die Polychromie 

 des Silbers.' 



*" E. F. Burton (Longmans, Green & Co., London) (1916). 'The Physical 

 Properties of Colloidal Solutions.' 



" Rayleigh, Phil. Ma;j., 35, 378 (1918). 'Scattering of Light.' 



12 W. D. Bancroft, Jour. Phys. Chem., 22, 601 (1918). ' Colour of Colloids.' 



" Haas, Ann. d. Physik., 57, 7, 568 (1918). ' Die Beziehungserscheinungen, 

 welche an einer grossen Anzahl unregelmassig verstreuter Offniingen oder 

 undurchlassigen Teilchen auftreten.' 



Gelatin. 



In the dry plate the gelatin is not only a protective colloid for 

 the preparation of the emulsion, an adhesive substance for attaching 

 the sensitive substance to the glass and a photochemical sensitiser 

 of the silver haUde, but it plays a most important part in development. 

 Unless gelatin, or some other colloid, is present, strong reducing agents 

 such as alkaline developers will reduce sUver halides without previous 

 exposure to hght. In presence of gelatin, however, this reaction is 

 extremely slow^. 



The oxidation products of developers usually tan the gelatin — 

 make it less soluble so that an ordinary negative shows the pictures 

 in reUef. The parts containing most silver being lowest, and the 

 clear gelatin projecting to the greatest height'. 



Sheppard and EUiot have given an explanation of the reticulation 

 of the surface of photographic negatives. Their explanation is based 

 on Procters' work on the effects of acids and alkalis on the swelling of 

 gelatin^". 



The colloid chemistry of gelatin is discussed in detail by Procter 

 in the First Report. The most important later work on the subject 

 is that of C. R. Smith, who has studied the mutarotation of gelatin". 

 He showed that gelatm in solution may be in a sol form A, stable 

 above 35° C, or a gel form B, stable below 15° C. Between these 

 two temperatures the two forms eventually come into equilibrium 

 and this causes mutarotation. Form B is mucli more viscous than 

 form A, and a certain definite concentration of B is necessary to 

 produce gelatinisation. The authors' experiments show that probably 

 two molecules of the A form combine to form one molecule of the 

 B form. 



* W. Reinders & J. van Niewenburg. Koll. Zelt., 10, 36 (1912). ' Gelatine 

 und andere Koiloide als Verzogerer bei der Reduktion von Chlorsilber.' 



- Lumiere & Seyewitz, Bull. Soc. Chim., iii., 35, 14 (1906). ' Sur la composi- 

 tion de la gelatine insolubilisee spontaneraent dans I'obscurite.' 



