the easily absorbable secondary radiation from one square centimeter of the metal under the circumstances of the experiment; V is the number of ions produced in 1 c.c. by the intrinsic penetrating radiation from the whole box and lead screen; V' is the number of ions produced in 1 c.c. by the external radiation and the penetrating radiation excited by it.
By using a strong electrostatic field, an attempt was made to determine whether the ionizing agents for the intrinsic absorbable radiation were charged. These radiations were found certainly not to be of the β type and very probably to have a nature similar to that of the α rays. No radium emanation was able to be detected from the lead used.
From the constancy of the value of s for the different specimens of the same metal and on account of the variation in the values of α for the different metals, Campbell rightly concludes that there seems to be no doubt but that the ordinary metals are feebly radioactive. In some cases the experimental and theoretical curves agree so well that it would seem that the radiations are homogeneous.
Campbell has also investigated the radioactivity of the metals and their salts in a similar way. He finds that the emission of radiations is an atomic property and that salts prepared by totally different methods and from materials derived from different sources, produce the same ionizing effect. It is only the metal that produces any measurable ionizing effect. The following are some of the results: