(p. 46) The problem with rubber was that it wasn’t a very versatile material. Macintosh found, for example, that in very hot weather his raincoats would “sweat,” and in freezing conditions they would crack. The solution to this particular problem came, as ever with innovation, by accident. In 1839 a young American working in the Roxbury India Rubber Company in Roxbury, Massachusetts, was experimenting with his raw materials one day when he accidentally let a mixture of rubber and sulfur drop onto a hot stove. The next morning he saw that the rubber had charred, like leather, instead of melting. He correctly inferred that if he could stop the charring at the right point, he’d have rubber that might behave like waterproof leather. The sulfur had vulcanized (he coined the word) the rubber in such a way that it would retain its shape and elasticity over a wide range of temperatures. So now rubber could be hard or elastic, as required.
Burke, James. The Pinball Effect: How Renaissance Water Gardens Made the Carburetor Possible – and Other Journeys. Boston: Back Bay Books, 1997.
(Note: italics in original.)