278 , Mr. Wm. Thomson and Mr. F. Lewis on 



manganese have also a destructive effect on rubber, although 

 not to such an extent as the copper salts ; still it is im- 

 portant to india-rubber water-proof manufacturers, who 

 are anxious to remove all possible causes of ultimate 

 damage to their rubber or rubber composition covered 

 cloths, to observe that the lamp black, ivory black, or other 

 similar black, is free from the oxides of manganese, for 

 one of us has found that in samples of such black compounds 

 the heavy qualities sometimes contain a considerable 

 proportion of these compounds. 



The very curious nature of india-rubber is further 

 shown by studying the tables of the actions of different 

 chemical compounds on it. Some chemists have condemned 

 cloths containing oxides of chromium for rubber proofing 

 purposes, and they would regard the presence of even a 

 trace of chromate or bichromate of potash, as fatal, and 

 the presence of free chromic acid in cloth would be 

 regarded by them as an agent likely almost immediately 

 to oxidise and destroy the rubber. Our experiments have 

 shown that these bodies have little or no injurious effect 

 on rubber, even when employed in large quantities and in 

 concentrated solutions. It, therefore, leaves the logwood 

 chrome blacks as available for use in the dyeing of cloths 

 intended for rubber proofing. We observed a remarkable 

 property of the iodide of arsenic, which we found might be 

 used as a reagent to determine whether rubber sheet had 

 been cold or steam-vulcanised. When a solution of this 

 substance is put upon india-rubber, vulcanised by the cold 

 process, which consists in the application to the rubber of 

 a mixture of chloride of sulphur and bisulphide of carbon, 

 the chlorine from the chloride of sulphur, which remains 

 in combination with the rubber, liberates the iodine from 

 the iodide of arsenic solution, producing a dark stain, whilst 

 no such effect is produced by unvulcanized india-rubber, or 

 iby rubber which has been steam vulcanized. 



