402 Mr. Frederick Soddy [March 15, 



the main shaft, which presses against it. The rate at which it moves 

 downward is always proportional to the height of the nut above the 

 point of the cone, that is to the quantity of substance remaining un- 

 changed. If the screw does not turn, the nut descends exponentially 

 with the time, illustrating the exponential decay of activity of a 

 single radioactive substance when left to itself. On the other hand 

 if the screw is now turned at constant speed in the direction to screw 

 the nut up, the nut mounts according to the usual law of the re- 

 generation of a radioactive constituent after it has been separated, 

 to the height such that it is turned down by the cone as fast as it is 

 turned up by the screw. At this point it is in equilibrium, precisely 

 analogous to radioactive equililjrium, and does not move however 

 long the machine is driven. The rubber-tyred 7iut communicates its 

 turns by means of a roller and gear-wheels to the screiv of the next 

 unit above, in accordance with the requirement that the quantity 

 of one member changing is the same as that of the next member pro- 

 duced. Thus the fall of the nut by virtue of the cone turning it and 

 screwing it down represents - Aq Q ; theorise of the nut, by virtue 

 of the screw turning and screwing the nut up, represents + Ap P of 

 the general differential equation. The various radioactive constants 

 are imitated (1) coarsely, by using screws with threads of different 

 pitch ; (2) finely, by varying the speed of the driving cone with refer- 

 ence to that of the main shaft by a suitable arrangement. 



To imitate the frequent case where the period of the radioactive 

 substance is infinitely long compared to the time of the experiment, 

 the rubber-tyred disc may be arranged, by undoing a lock-nut, to 

 "free wheel" without turning the nut. This then corresponds to a 

 screw" of infinitely fine pitch, or to a radioactive substance of in- 

 finitely long period. The numerous cases of radioactive change dis- 

 cussed in the lecture w^ere imitated by graphs drawn by the machine. 



In addition to offering a consistent explanation of all the known 

 facts that had been accumulated in radioactivity, the theory of 

 atomic disintegration suggested a large number of new problems. 

 Only two of these original problems remain not yet completely 

 answered. One had to do with the nature of the ultimate product or 

 products of the disintegration of the atoms of the two primary 

 elements, uranium and thorium. This problem may be likened to 

 the task of trying to find a meteor after its flight, when its energy is 

 spent and nothing but the matter remains. Much indirect evidence 

 points to lead as the final product of uranium, although no direct 

 proof has been obtained, whereas for the case of thorium there 

 is still no hint of the answer. The other had reference to the 

 origin of radium. This element in the intensity i of its activity, and, 

 therefore, in the rapidity of its disintegration, resembles the short- 

 lived active constituents uranium X and thorium X, whilst in the 

 apparent permanence of its activity it resembles the primary radio- 

 elements. Even the first rough estimates indicated that the period of 



