As a typical example of the evidence, from which it is deduced that one substance is the parent of another, we will consider the connection of the two products Th X and the thorium emanation. It has been shown (section 154) that after the separation of Th X from a thorium solution, by precipitation with ammonia, the precipitated thorium hydroxide has lost to a large extent its power of emanating. This cannot be ascribed to a prevention of escape of the emanation produced in it, for very little emanation is observed when a current of air is drawn through the hydroxide in a state of solution, when most of the emanation present would be carried off. On the other hand, the solution containing the Th X gives off a large quantity of emanation, showing that the power of giving off an emanation belongs to the product Th X. Now it is found that the quantity of emanation given off by the separated Th X decreases according to an exponential law with the time, falling to half value in four days. The rate of production of emanation thus falls off according to the same law and at the same rate as the activity of the Th X measured in the ordinary manner by the α rays. Now this is exactly the result to be expected if the Th X is the parent of the emanation, for the activity of Th X at any time is proportional to its rate of change, _i.e._, to the rate of production of the secondary type of matter by the emanation in consequence of a change in it. Since the rate of change of the emanation (half transformed in 1 minute) is very rapid compared with the rate of change of Th X, the amount of emanation present will be practically proportional to the activity of the Th X at any instant, _i.e._, to the amount of unchanged Th X present. The observed fact that the hydroxide regains its power of emanating in the course of time is due to the production of fresh Th X by the thorium, which in turn produces the emanation. In a similar way, excited activity is produced on bodies over which the emanation is passed, and in amount proportional to the activity of the emanation, _i.e._, to the amount of the emanation present. This shows that the active deposit, which gives rise to the phenomenon of excited activity, is itself a product of the emanation. The evidence thus seems to be conclusive that Th X is the parent of the emanation and that the emanation is the parent of the deposited matter. =194. Chemical and Physical properties of the active products.= Each of these radio-active products is marked by some distinctive chemical and physical properties which differentiate it from the preceding and succeeding products. For example, Th X behaves as a solid. It is soluble in ammonia, while thorium is not. The thorium emanation behaves as a chemically inert gas and condenses at a temperature of −120° C. The active deposit from the emanation behaves as a solid and is readily soluble in sulphuric and hydrochloric acids and is only slightly soluble in ammonia.