Chemical element of metals. Cu comes from Cuprum, Fe comes from Ferrum, Ag from Argentum. The symbols were not followed by a period (full stop) as with abbreviations. Later chemical elements were also assigned unique chemical symbols, based on the name of the element, but not necessarily in English. For example, sodium has the chemical symbol 'Na' after the Latin "natrium". The same applies to "W" (wolfram) for tungsten, "Fe" (ferrum) for iron, "Hg" (hydrargyrum) for mercury, "Sn" (stannum) for tin, "K" (kalium) for potassium, "Au" (aurum) for gold, "Ag" (argentum) for silver, "Pb" (plumbum) for lead, "Cu" (cuprum) for copper, and
19 (number) 19 (number) 19 (nineteen) is the natural number following 18 and preceding 20. It is a prime number. In English speech, the numbers 19 and 90 are sometimes confused, as they sound very similar. 19 is the 8th prime number. 19 is the seventh Mersenne prime exponent. 19 is the maximum number of fourth powers needed to sum up to any natural number. It is the fourth value of g(k). 19 is the lowest prime centered triangular number, a centered hexagonal number and a Heegner number. The only non-trivial normal magic hexagon contains 19 hexagons (the other being 1). In