RUBBER PLANTING 233 



for a longer period than thin ones. The proportion of 

 sulphur added to the rubber varies according to the 

 nature of the articles to be manufactured. For ordinary 

 goods the quantity is- about 7 to 10 per cent, of the 

 amount of rubber. 



Bath Process. 



In Hancock's bath process, which is comparatively 

 little used, small objects are immersed in molten sulphur 

 for a period of two or three hours at a temperature 

 of I3OC. to I35C. Test pieces of rubber, of similar 

 thickness and composition to the articles to be vul- 

 canised, are placed in the same bath and removed at 

 intervals. From the appearance of these the operator 

 is able to judge when the process of vulcanisation is 

 complete. 



Cold Process. 



The cold process, discovered by Parkes, can only be 

 used for vulcanising very thin sheet. This is due to the 

 extreme rapidity of the process, which is such that if it 

 were applied to thick rubber, the surface of the object 

 would be converted into vulcanite before the interior 

 was properly vulcanised. The reagent employed is 

 sulphur monochloride, prepared by passing dry chlorine 

 over heated sulphur. This substance is very active 

 chemically, and is decomposed by water. As a solvent, 

 carbon bisulphide is almost universally adopted. Carbon 



