FIRE

rapid burning of combustible material with the evolution of heat and usually accompanied by flame. It is one of the human race's essential tools, control of which helped start it on the path toward civilization. The original source of fire undoubtedly was lightning, and such fortuitously ignited blazes remained the only source of fire for aeons. For some years Peking man, about 500,000 BC, was believed to be the earliest unquestionable user of fire; evidence uncovered in Kenya in 1981 and in South Africa in 1988, however, suggests that the earliest controlled use of fire by hominids dates from about 1,420,000 years ago. Not until about 7000 BC did Neolithic man acquire reliable fire-making techniques, in the form either of drills, saws, and other friction-producing implements or of flint struck against pyrites. Even then it was more convenient to keep a fire alive permanently than to reignite it. in gems, rapidly changing flashes of colour seen in some gems, such as diamonds. Some minerals show dispersion; that is, they break incident white light into its component colours. The greater the separation between rays of red light (at one end of the visible spectrum) and rays of violet light (at the other end), the greater the dispersion and the greater the fire, because the separate coloured rays produce this phenomenon. In properly cut gemstones with strong dispersion, white light entering through the crown will be reflected and returned through the crown as coloured light; improper cutting, however, reduces reflection and, hence, the stone's fire.