MOLECULAR AND MACROMOLECULAR STRUCTURE 191 



and vary widely. The methods employed do not in theory all measure the 

 same quantity and even when the actual measurements themselves are 

 precise, their theoretical bases are much in dispute. An attempt has been 

 made to make sense out of this unsatisfactory situation by Alexander and 

 Hudson (1954) and reference may be made to their discussion since it 

 would carry us far beyond the present intention to attempt to review the 

 work here. The broad conclusion of the physicochemical work based on 

 the penetration of various molecules into fibres is that only a small pro- 

 portion of the fibre (10-30%) is inaccessible to small molecules and can in 

 this sense be regarded as crystalline. " Inaccessibility " is a concept not 

 necessarily equivalent to " crystallinity " in the X-ray sense, since 

 crystallites capable of giving a Bragg reflection may be partly or wholly 

 penetrated by some reagents. 



From a suitable X-ray pattern an estimate of the crystalline/amorphous 

 ratio may also be calculated from measurements of the amount of radiation 

 scattered in the form of discrete reflections and that scattered into diffuse 

 reflections. In favourable cases, e.g. rubber, this theory has been used to 

 determine the ratio in the stretched and unstretched state, and has helped 

 to confirm that the long-range elasticity is based on the stretching of 

 randomly-coiled chains which may then crystallize when held in the 

 stretched condition. A similar study carried out on a suitable keratin fibre 

 would be of great value. However, the vague patterns render the project 

 well-nigh impossible since there is considerable overlap of diffuse and 

 discrete reflections. 



An inspection of a typical a-pattern (Plate 1) reveals the presence of 

 considerable amounts of diffuse scattering due to amorphous material. 

 This is seen principally as a broad diffuse halo centred about an average 

 spacing of 4-5 A (Astbury and Street, 1931; Astbury and Woods, 1933). 

 No quantitative estimate of the amount of this reflection has been made. 

 Qualitatively it appears considerable and seems to suggest that more 

 than half of the fibre substance is amorphous. During the stretching of a 

 hair, the appearance of the pattern is little affected in the range of 0-20% 

 extension; after that certain reflections specifically associated with the 

 a-type structure fade, although there is no perceptible change in the 

 distribution and intensity of the diffuse scattering. When a fibre which has 

 been stretched beyond 50% is steamed, in order to induce recrystallization 

 of the /S-form, the /^-pattern appears against a background of diffuse 

 scattering very similar in intensity to that noted in the unstretched fibre. 

 This shows that stretching does not itself produce an increased amount of 

 the fibre substance in an ordered form (a or /S) and suggests that the same 

 well-ordered regions give rise both to the a- and the /S-pattern according to 

 their extension. 



Measurements of the birefringence of hairs (Barnes, 1933; Mercer, 



