§ =88.= Radium is an element resembling calcium, strontium, and barium in chemical properties; its atomic weight was determined by Mme. Curie, and found to be about 225, according to her first experiments; a redetermination gave a slightly higher value, which has been confirmed by a further investigation carried out by Sir T. E. Thorpe.[103] Radium gives a characteristic spectrum, and is intensely radioactive. It should be noted that up to the middle of the year 1910 the element radium itself had not been prepared; in all the experiments carried out radium salts were employed (_i.e._, certain compounds of radium with other elements), generally radium chloride and radium bromide. In that year, however, Mme. Curie, in conjunction with M. Debierne, obtained the free metal. It is described as a white, shining metal resembling the other alkaline earth metals. It reacts very violently with water, chars paper with which it is allowed to come in contact, and blackens in the air, probably owing to the formation of a nitride. It fuses at 700° C., and is more volatile than barium.[104] [103] See Sir T. E. THORPE: “On the Atomic Weight of Radium” (Bakerian Lecture for 1907. Delivered before the Royal Society, June 20, 1907), _Proceedings of the Royal Society of London_, vol. lxxx. pp. 298 _et seq._; reprinted in _The Chemical News_, vol. xcvii. pp. 229 _et seq._ (May 15, 1908). [104] Madame P. CURIE and M. A. DEBIERNE: “Sur le radium métallique,” _Comptes Rendus hebdomadaires des Séances de l’Académie des Sciences_, vol. cli. (1910), pp. 523-525. (For an English translation of this paper see _The Chemical News_, vol. cii. p. 175.) The Radioactivity of Radium.