28o Mr. Wm. Thomson and Mr, F. Lewis on 



stretching, it seemed to be stronger and more elastic than 

 the original rubber. With a view to finding the effect of a 

 minute quantity of sulphuric acid on india-rubber, we 

 soaked a piece of thin sheet rubber in solutions containing 

 j^th, I, 2, and 5 per cent of sulphuric acid respectively, until 

 the solution had thoroughly penetrated the sheet, which then 

 appeared white. This was allowed to dry, and heated for 

 some days to a temperature of 140° Fah,, but the small 

 quantity of acid exerted no injurious effect on the rubber. 

 When, however, the rubber was taken from the strongest 

 sulphuric acid solution containing 10 per cent of acid, dried 

 and heated to 212° Fah. in a similar manner, it was soon 

 destroyed. The sheets left in the cold for a month were 

 likewise all sound except that placed in nitric acid, which 

 was rendered quite friable, the elasticity having entirely gone 



Effects of Over Mastication on the life of India-rubber. 



It has been so often asserted by india-rubber manufac- 

 turers that over mastication seriously damages india- 

 rubber and leads to its rapid decay afterwards, that it seems 

 extremely heterodox to say anything to the contrary. Still 

 the results which we have obtained lead us to this opinion. 

 In preparing rubber for spreading on cloth in the manufac- 

 ture of water-proof fabrics, it is first passed between heavy 

 rollers with a stream of water pouring over it, to remove 

 stones or dirt which might be associated with it ; it is then 

 dried thoroughly and masticated between large heavy smooth 

 rollers for a few minutes, these being gradually screwed 

 closer and closer together during the operation. This 

 softens the rubber, and enables it afterwards to be brought 

 into a uniform solution when mixed with the naphtha, and 

 the longer it is masticated the less naphtha is afterwards 

 required to bring it to the proper uniform consistency for 

 spreading. It is highly improbable that over-mastication 

 would ever be done in practice, because the workman has 



