Natural rubber is generally soft, has high water absorption tendency. So, it is treated with an element X, which introduced cross links into the structure of rubber and hence improved its properties. This process is called vulcanisation.
The element X can be

Vulcanisation is a chemical process in which rubber is heated with sulphur at 140 –  160°C. The process involves the formation of cross links between long rubber molecules so as to  achieve improved elasticity, tensile strength, viscosity, hardness and weather resistance.