Physical Properties of Thorium

The metal obtained by Nilson, which was not free from oxide, was a dark grey, soft, shining powder consisting of microscopic, hexagonal plates; it had a density of 11.0, whence that of the pure metal was calculated to be 11.10. Von Bolton's metal had a density of 11.32 and, after rolling arid heating, 12.16. The specific heat of thorium, according to the mean of six determinations made by Nilson, is 0.02787. The metal burns brilliantly to the oxide when heated in the air, though less easily in the form of ribbon than in powder. Its heat of combustion is given by the equation

Th + O₂ = ThO₂ + 326,000 ± 0.5 per cent, calories.

It melts at about 1450° C. in a vacuum (von Bolton); von Wartenberg's metal, however, containing about 2 per cent, of carbide melted at 1700° C. In colour, softness, and ductility thorium resembles platinum. It combines, on heating, with the halogens, sulphur, hydrogen, and nitrogen; it dissolves with difficulty in hydrochloric acid, and its reaction with nitric acid soon comes to a standstill, owing to the metal becoming passive; it dissolves rapidly in aqua regia, but alkalis are without action upon it. Colloidal thorium has been obtained by Wedekind and Baumhauer. The radioactivity of thorium will be dealt with in a special section.

The most intense lines in the arc and spark spectra of thorium are as follow:

Arc: 3188.33, 3511.76, 3741.36, 4019.29, 4382.02, 4391.29, 4752.60, 4863.38, 4919.99, 5017.39, 5049.93, 5989.22, 6462.83.

Spark: 2441.38, 3221.40, 3290.73, 3300.63, 3313.87, 3507.72, 3538.90, 4019.30, 4382.10, 4391.30.

The most persistent spark lines, which should be looked for when seeking traces of thorium, are as follow:

2549.7, 2554.8, 2555.3, 2870.5, 3290.7, 4382.1, 4391.3.