612 REPORT — 1903. 



3. Jie2?07-t of the Committee on securing DiUy-free Alcohol for Scientific 

 Research. — See Reports, p. 170. 



4, 21ie Cause of the Lustre prodziced on Mercerising Cotton under Tension. 

 By Julius Hubner, F.C.S., and William J. Pope, F.R.S. 



It is generally supposed that the productiou of a lustre on treating stretched 

 cotton yarn with strong caustic soda is conditioned by only two factors — namely, 

 by the simultaneous swelling and shrinking of the fibres. The authors show, 

 however, that a third effect is essential to the production of any appreciable silky 

 lustre : this consists in an uncoiling of the naturally twisted ribbon constituting 

 the cotton-fibre. 



On immersing a loose cotton-fibre in strong caustic soda on the microscope 

 stao-e, it is seen to rapidly untwist, to swell, and at the same time to shorten in 

 length : the untwisting generally continues until the natural twist has nearly 

 completely disappeared, after which the fibre presents the appearance of a round 

 irregularly curved rod with a comparatively smooth surface. If the fibre is fixed 

 at one end and treated with caustic soda, it twists either to the right or to the 

 left, according as it was originally coiled towards the left or towards the right ; in 

 the most generally occurring case, that, namely, in which the fibre is coiled partly 

 to the right and partly to the left, the untwisting attending the treatment with 

 soda takes place first towards the left and then towards the right, or vice versa. 

 If the fibre is prevented from contracting by being held nt the two ends in a 

 stretched condition it still untwists when treated with soda; since, however, the 

 whole of the unt%visting does not take place simultaneously, the untwisting of 

 one part causes another part, which has already become unwound and attained 

 the condition of a gelatinous rod, to become tightly twisted in the opposite 

 direction to its original twist. 



The stretched fibre thus again acquires a corkscrew-like appearance, part of the 

 twist being right- and part left-handed, with the diSerence, however, that whilst 

 the raw fibre forms a twisted ribbon creased or folded at the turns, treatment 

 with soda converts it into a rod of circular cross-section which has been twisted 

 whilst in a gelatinous state. The twisting of the fibre under these conditions 

 results in the production on the rounded surface of spiral ridges possessing smooth 

 curved contours, which reflect the light at all angles of incidence and reflection 

 iust as do the coils of a polished corkscrew. The fibre, therefore, becomes 



lustrous. n, • J 



The hio-h degree of transparency possessed by the cotton-fibre introduces 

 difiiculties into the microscopic examination of the changes referred to above. But 

 although the fibre is amorphous, it is doubly refracting owing to internal strain ; 

 the authors therefore find it convenient to conduct the microscopic examination of 

 the fibre between crossed Nicol prisms, and to accentuate the difl'erence in tint of 

 the various parts l)y introducing a one-eighth wave-length retardation plate of 

 mica between the Nicols in such a way that its principal directions make an angle , 

 of 45° with those of the prisms. This enables the internal canal, cracks in the 

 surface, and differences in thickness to be made out with great ease. The 

 correctness of the explanation now given of the lustre is shown by a series of 

 photomicrographs taken in natural colours in elliptically polarised light under the 

 conditions just referred to. The authors have to thank their colleague, Mr. Charles 

 W. Gamble, Director of the Photographic Department in the Manchester Municipal 

 School of Technology, for having assisted the work by the production of these 

 photographs. A further confirmation of the correctness of the conclusions now 

 arrived at is afforded by the observation that whilst cotton-fibres mercerised loose 

 have a practically circular cross-section, fibres treated under tension with soda 

 show cross-sections shaped like polygons with rounded corners. 



An independent proof of the authors' conclusions that the untwisting of the 

 fibre is as essential a factor in the production of the gloss as are the swelling and 



