81 



indiflference to temperature, and its chemical inertia, are in marked 

 contrast to gelatine, and are the main characters which render it of 

 such importance as a support. Any discussion of the photo-chemical 

 reactions involved in the preparation or utilisation of collodion plates 

 or papers would be redundant, as they are of the same general character 

 as occur in all photographic processes, and are dealt with elsewhere. 



A very full account of the history and preparation of collodion 

 and its application to photography will be found in Vol. 2 of Worden's 

 " Nitro-cellulose Industry," 1911, pages 827 to 897, where copious 

 references are given to both patents and Uterature. 



Very little exact information exists as to the specific characteristics 

 of photographic collodion. It is obvious that the character of any 

 solution will depend upon the nature of the nitro-cellulose used, the 

 solvents being under comparatively perfect control. The exact 

 nature of the nitro-cellulose depends upon two factors, namely, the 

 raw material used for nitration, and the constitution of the resultant 

 nitro-cellulose, w^hicli latter naturally depends upon conditions and 

 details of the nitration process. The permissible nitrogen content 

 for the production of a photographic collodion lies somewhere between 

 11 per cent, and 12 per cent., and generally about 11-5 per cent. It 

 is found that quite small vai-iations of nitrogen content may involve 

 large variations m physical character, such as, refractive index, 

 viscosity, water compatibiUty, &c. Hence the mere nitrogen content, 

 within the limits mentioned, does not in any w^ay constitute a guide 

 as to the suitability, or otherwise, of a nitro-cotton. Many attempts 

 have been made to devise methods which would yield more positive 

 results, but with comparatively small success; the behaviour of the 

 nitrated fibre towards polarised Ught has been investigated by several 

 Avorkers, and a rough indication of the degree of nitration of the 

 cotton can be obtained in this manner. The greatest value of the 

 method so far is that it clearly differentiates, first, unnitrated fibres ; 

 and second, fibres of varying degrees of nitration. It is now realised 

 that the usefuhiess of polarised light will be greatly extended when 

 the results obtained by its use are correlated, not only Avith the nitrogen 

 content, but Avith the physical properties, such as viscosity, refractive 

 index, &c., which are of much greater significance than the nitrogen 

 content in determining the properties of the collodion as far as its 

 photographic utUity is concerned. 



The lack of knowledge as to the constitution of the nitro-cellulose 

 molecule is a bar to more exact information regarding the behaviour 

 of nitro-cellulose solutions, as collodion, in the various processes for 

 for which it is used in photogi'aphy. Results of an anomalous 

 character are frequent in aU investigations, and although there is now- 

 available a large volume of observations, they cannot yet be exactly 

 correlated, nor is the information available which explains their 

 occurrence. The effect of dr3ing the cotton before nitration, and 

 especially the temperature and treatment to which it is subjected 

 before the actual nitration takes place, has a profound influence on 

 the physical properties of the resultant nitro-cellulose. Further, 

 both time and temperature modify the nitrated cotton. It would 

 appear that all nitro-cottons undergo a process of denitration to a 



X 11454 V 



