In the above calculations it is assumed, as a first approximation, that the β rays from C and E have the same average velocity. This is probably not accurately the case, but the above number certainly serves to fix the order of magnitude of the period of the product D. This calculation is confirmed by observations to be given later on the amount of D and E in old radium. It may be of interest to mention that the writer calculated the period of radium F by a similar method, before its value was experimentally determined, and found that F should be half transformed in about one year. This is not very different from the experimental value of 143 days found later. In addition, it was assumed in the calculation that the α particles from C and F were projected with the same velocity, and in consequence produced the same amount of ionization. In practice, however, it is found that the α particle of F is absorbed in about half the distance of the α particles of C, and in consequence produces only about half of the ionization of the latter. If this correction were made, the calculated period for half transformation would be six months instead of one year. A table of the transformation products of radium, together with some of their physical and chemical properties, is given below. Transformation Time to be Rays Chemical and Products half Physical transformed Properties Radium 1200 years α rays — Emanation 3·8 days α rays Chemically inert gas; condenses at −150° C. Radium A 3 mins. α rays Behaves as (active deposit solid; of rapid deposited on change) the surface of bodies; concentrated on cathode in electric field. Soluble in strong acids; volatile at a white heat. B is more volatile than A or C. :: B (same) 21 mins. no rays Same :: C (same) 28 mins. α, β, γ rays Same