TABLE OF FIXED POINTS. ──────────────────┬────────────────────┬───────┬────── Substance. │Physical Condition. │ Deg. │ Deg. │ │ Cent. │ Fahr. ──────────────────┼────────────────────┼───────┼─────── Water (ice) │ At Melting Point │ 0 │ 32 Water │ ” Boiling ” │ 100 │ 212 Aniline │ ” ” ” │ 184 │ 363 Naphthalene │ ” ” ” │ 218 │ 424 Tin │ ” Melting ” │ 232 │ 449 Lead │ ” ” ” │ 327 │ 620 Zinc │ ” ” ” │ 419 │ 786 Sulphur │ ” Boiling ” │ 445 │ 833 Antimony │ ” Melting ” │ 631 │ 1167 Aluminium │ ” ” ” │ 657 │ 1214 Common Salt │ ” ” ” │ 800 │ 1472 Silver (in air) │ ” ” ” │ 955 │ 1751 Silver (free from │ │ │ oxygen) │ ” ” ” │ 962 │ 1763 Gold │ ” ” ” │ 1064 │ 1947 Copper (in air) │ ” ” ” │ 1064 │ 1947 Copper (Graphite │ │ │ covered) │ ” ” ” │ 1084 │ 1983 Iron (pure) │ ” ” ” │ 1520 │ 2768 Palladium │ ” ” ” │ 1549 │ 2820 Platinum │ ” ” ” │ 1755 │ 3190 ──────────────────┴────────────────────┴───────┴─────── It is necessary to point out that the figures given in the table refer only to pure substances, and that relatively small quantities of impurities may give rise to serious errors. The methods by which the physical condition to which the temperatures refer may be realised in practice will be described in the succeeding chapter. =National Physical Laboratory Scale.=—Exact agreement with regard to fixed points has not yet been arrived at in different countries, and an effort to co-ordinate the work of the National Physical Laboratory, the United States Bureau of Standards, and the Reichsanstalt, with a view to the formation of an international scale, was interrupted by the war. In 1916 the National Physical Laboratory adopted a set of fixed points on the Centigrade thermodynamic scale, in conformity with which all British pyrometers have since been standardised. It will be seen that the figures differ very slightly from those given in the previous table, which represent the average results of separate determinations in different countries. NATIONAL PHYSICAL LABORATORY SCALE (1916).