The term “active deposit” can, however, only be used when the matter is spoken of as a whole; for it will be shown later that the matter, under ordinary conditions, is complex and contains several constituents which have distinctive physical and chemical properties and also a distinctive rate of change. According to the theory advanced in section 136, we may suppose that the emanation of thorium, radium, and actinium is unstable and breaks up with the expulsion of an α particle. The residue of the atom of the emanation diffuses to the sides of the vessel or is removed to the negative electrode in an electric field. This active deposit is in turn unstable and breaks up in several successive stages. The “excited activity” proper is the radiation set up by the active deposit in consequence of the changes occurring in it. On this view, the emanation is the parent of the active deposit in the same way that Th X is the parent of the emanation. The proportionality which always exists between the activity of the emanation and the excited activity to which it gives rise, is at once explained, if one substance be the parent of the other. =179. Decay of the excited activity produced by thorium.= The excited activity produced in a body after a _long_ exposure to the emanations of thorium, decays in an exponential law with the time, falling to half value in about 11 hours. The following table shows the rate of decay of the excited activity produced on a brass rod. Time in Current hours 0 100 7·9 64 11·8 47·4 23·4 19·6 29·2 13·8 32·6 10·3 49·2 3·7 62·1 1·86 71·4 0·86 The results are shown graphically in Fig. 64, Curve _A_. [Illustration: Fig. 64.] The intensity of the radiation _I_ after any time _t_ is given by $$ \frac {I} {I₀} = e^{–λt} $$, where λ is the radio-active constant. The rate of decay of excited activity, like that of the activity of other radio-active products, is not appreciably affected by change of conditions. The rate of decay is independent of the concentration of the excited activity, and of the material of the body on which it is produced. It is independent also of the nature and pressure of the gas in which it decays. The rate of decay is unchanged whether the excited activity is produced on the body with or without an electric field.