Section III, 1919 [159] Trans. R.S.C. 



Absorption of Light by Thin Films of Rubber 

 By E. R. I. Pratt, M.A., University of Toronto 



Presented by E. F. Burton, F. R.S.C. 



(Read May Meeting, 1919.) 



Considerable work on the influence of light on rubber has been 

 done by Victor Henry. In his investigations he experimented with 

 pure unvulcanized rubber, and with vulcanized rubber cured in differ- 

 ent ways. 



The pure rubber as obtained from Para sheets was exposed to the 

 rays of a mercury vapour lamp for several hours. The samples ex- 

 posed in this manner showed considerable change in colour and lost 

 their elasticity, being readily torn. The change in the rubber appeared 

 to depend on the colour of the original sheet, the dark brown sample 

 being altered at the surface only, while the "yellow plantation" was 

 changed through a considerable thickness. The vulcanized samples 

 had to be left for a much longer period than that of the unvulcanized 

 before any apparent change was noted. 



An effort was also made to determine what particular part of 

 the spectrum was most injurious to the rubber. He arrived at the 

 conclusion that the rays below 3000 A. are the most active in this 

 respect. Some of the rays between 4000 and 3000 A. were strongly 

 absorbed, but the most refrangible rays are the particular agents 

 which alter the rubber, the unvulcanized samples being altered most. 



The foregoing results were confirmed during the course of the 

 preliminary work in the present investigation, and prove conclusively 

 that light has a very great influence on rubber and that certain wave 

 lengths are more active than others in this respect. Beyond ascertain- 

 ing the effect of light on rubber in changing its appearance, elasticity 

 and general physical properties, no attempt was made, by previous 

 investigators, to obtain in a quantitative way the absorption of light 

 for different parts of the spectrum. 



The purpose of this investigation was to ascertain in a quantita- 

 tive manner the amount of light absorbed by rubber and how it varied 

 for different wave lengths. 



Pure Para rubber sheets were obtained, light brown in colour and 

 several millimetres thick. Efforts were made to cut the sheets into 

 thinner ones, for this purpose a microtome being used, but owing to 

 the great elasticity of the rubber this method had to be abandoned as 



