This emanation produces excited activity on surrounding objects, and at diminished pressure the emanation produces a uniform distribution of excited activity in the enclosure containing the emanation. The excited activity falls to half value in 41 minutes. Debierne observed that the distribution of excited activity was altered by a strong magnetic field. The experimental arrangement is shown in Fig. 71A. The active matter was placed at _M_, and two plates _A_ and _B_ were placed symmetrically with regard to the source. On the application of a strong magnetic field normal to the plane of the paper, the excited activity was unequally distributed between the plates _A_ and _B_. The results showed that the carriers of excited activity were deviated by a magnetic field in the opposite sense to the cathode rays, _i.e._ the carriers were positively charged. In some cases, however, the opposite effect was obtained. Debierne considers that the excited activity of actinium is due to “ions activants,” the motion of which is altered by a magnetic field. Other experiments showed that the magnetic field acted on the “ions activants” and not on the emanation. [Illustration: Fig. 71A.] The results of Debierne thus lead to the conclusion that the carriers of excited activity are derived from the emanation and are projected with considerable velocity. This result supports the view, advanced in section 190, that the expulsion of α particles from the emanation must set the part of the system left behind in rapid motion. A close examination of the mode of transference of the excited activity by actinium and the emanation substance is likely to throw further light on the processes which give rise to the deposit of active matter on the electrodes. Footnote 269: M. and Mme. Curie, _C. R._ 129, p. 714, 1899. Footnote 270: Rutherford, _Phil. Mag._ Jan. and Feb. 1900. Footnote 271: As regards date of publication, the priority of the discovery of “excited activity” belongs to M. and Mme. Curie. A short paper on this subject, entitled “Sur la radioactivité provoquée par les rayons de Becquerel,” was communicated by them to the _Comptes Rendus_, Nov. 6, 1899. A short note was added to the paper by Becquerel in which the phenomena of excited activity were ascribed to a type of phosphorescence. On my part, I had simultaneously discovered the emission of an emanation from thorium compounds and the excited activity produced by it, in July, 1899. I, however, delayed publication in order to work out in some detail the properties of the emanation and of the excited activity and the connection between them. The results were published in two papers in the _Philosophical Magazine_ (Jan. and Feb. 1900) entitled “A radio-active substance emitted from thorium compounds,” and “Radio-activity produced in substances by the action of thorium compounds.” Footnote 272: Rutherford, _Phil. Mag._ Feb. 1900. Footnote 273: