195.  In a recent paper, Giesel (Ber. d. D. Chem. Ges. No. 3, p. 775, 1905) has shown that the bright lines are due to didymium, which is present as an impurity. Exposure of didymium to the radium rays also causes the appearance of the lines.

196.  Wiedemann and Schmidt, Wied. Annal. 59, p. 604, 1895.

197.  Wiedemann, Phys. Zeit. 2, p. 269, 1901.

198.  Elster and Geitel, Annal. d. Phys. 69, p. 673, 1899.

199.  Willons and Peck (Phil. Mag. March, 1905) found that under some conditions, especially for long sparks, the rays of radium hindered the passage of the spark.

200.  Hemptinne, C. R. 133, p. 934, 1901.

201.  Himstedt, Phys. Zeit. p. 476, 1900.

202.  Henning, Annal. d. Phys. p. 562, 1902.

203.  Kohlrausch and Henning, Verh. Deutsch. Phys. Ges. 6, p. 144, 1904.

204.  Kohlrausch, Verh. Deutsch. Phys. Ges. 5, p. 261, 1904.

205.  P. Curie, C. R. 134, p. 420, 1902.

206.  Becquerel, C. R. 136, p. 1173, 1903.

207.  Becquerel, C. R. 133, p. 199, 1901.

208.  P. Curie, Société de Physique, March 2, 1900.