Supplementary Chapter 321 



tion spectra of cellulose and derivatives, such as can be obtained 

 in the form of transparent films, and the relative diathermancy 

 of these at the same time we are re-determining the constants 

 forjdensity in air and in neutral liquid media. Generally we 

 are aiming at a systematic exposi of these physical characteristics 

 in terms of quantitative data. These may be expected to throw 

 light on the stillbbscure problems of ultimate constitution. 



There is another and complementary problem of constitu- 

 tional form or structure, belonging to cellulose as a typical 

 colloid. Investigations of certain physical phenomena, and 

 provisionally limited to these, are calculated to reveal some 

 definite picture of the proximate structure of apparently 

 structureless forms, such as represented by transparent homo- 

 geneous films. 



Thus, W. Brown (Biochem. 7.1915, 9, 591) has reinvestigated 

 the whole question of semi-permeable membranes from this 

 point of view in a particular study of the ' collodion ' film. 

 From the author's opening bibliography it may be concluded 

 that this invaluable addition to biological method has not yet 

 emerged from its empirical limitations ; but with the control of 

 graduated osmotic permeability established by this systematic 

 investigation of the c nitro-cellulose ' film, the author has not 

 only contributed a perfected instrument for biophysical investi- 

 gations, but a means of investigating the proximate structure 

 of typical colloids in that critically important region where the 

 aggregation of structural units merges into the relationship of 

 these to the ultimate constitutional groups. It is now estab- 

 lished that films formed from collodion under the conditions 

 of controlled evaporation of the (alcohol-ether) solvent, to the 

 air-dry condition, are specifically and permanently modified by 

 immersion in aqueous alcohol of definite ' strength,' i.e. 

 alcohol : water ratio, in respect of osmotic permeability. 



The graduation of specific permeability is in fact reduced 



