742 



Prof. E. G. Coker on the 



the desirable features of glass, with the additional advantage 

 of being easily cut into shape without suffering any injury 

 of its elastic or optical properties. 



Xylonite is a preparation of nitrocellulose of widely ex- 

 tended use, and it can be obtained in sheets several square 

 feet in area free from initial stress except at the edges, so 

 that it shows no colour effect in polarized light. 



It is not quite so transparent as glass, and it is usually 

 slightly tinted, but this is not found to be a disadvantage in 

 practice. 



Specimens may be cut from the sheet by ordinary wood 

 and metal working tools, and with a little practice airy object 

 capable of being represented in a plane may be fashioned 

 with ease. The material is fairly isotropic in character, and 

 although a sheet appears to show a distinct grain, this is 

 apparently a mere surface effect due to the sheet being cut 

 from a plastic slab ; it is afterwards subjected to treatment 

 which renders it semielastic, and its surface is finally polished 

 before use. 



Xylonite is much more compressible than glass, and it 

 possesses the useful property of not readily breaking under 

 stress. Up to a stress of about 4000 lbs. per square inch it 

 may be subjected to repeated stresses without injury or 

 change of its optical character, but beyond this it shows 

 signs of residual stress when the load is removed. 



Its elastic behaviour under stress is not so perfect as that 

 of glass, and the values of Young's modulus and Poisson's 

 ratio are very different. 



This will be readily seen when the results of experiments 

 on specimens of both materials are compared. 



The following table gives the values obtained in a tension 

 specimen of xylonite 8 ins. long between measuring-points, 

 0*49 in. wide and 0*123 in. thick. 



Table I. 



Load, 

 pounds. 



i 



-r, , Contraction 

 .Extensions, ,. • ., . - r .- cc 



'-r..np > of width, Differences, 

 ins. Ihiierenees. 



ins. 







40 



80 



120 



160 



120 



80 



40 







w -ml 



•035 ™§ 

 .n?i '019 

 •055 01b 



•03? :^ 



. oo4 



•00085 -00 045 

 000f -00045 



•0017 '000 4 

 •00 4 -O 003 



•0001 "O 004 



•00055 -0°0 ±5 



•ooof -ooo 45 



