"Ideas may be compared to the extension and solidity of matter, and impressions, especially reflective ones, to colours, tastes, smells and other sensible qualities. Ideas never admit of a total union, but are endowed with a kind of impenetrability, by which they exclude each other, and are capable of forming a compound by their conjunction, not by their mixture. On the other hand, impressions and passions are susceptible of an entire union; and like colours, may be blended so perfectly together, that each of them may lose itself, and contribute only to vary that uniform impression, which arises from the whole. Some of the most curious phaenomena of the human mind are derived from this property of the passions."

David Hume, A Treatise of Human Nature, Book II, Part II, Section VI.

Web presentations

• Rubber: Macintosh Mill, Manchester. Important Web presentation by Ian Miller on behalf of OxfordArchaeology. Not sure all the detail here is corroborated by other sources (Hazel)
• Bouncing-Balls.com - Everything you have wanted to know about rubber. Extensive and authoritative presentation by John Loadman
• Biography of Charles Goodyear, Scientific American Supplement No. 787, 1901
• Article on Rubber in the Wikipedia
• Notes on rubber and empire
• History of natural rubber