A History of Science — Volume 5

by Henry Smith Williams; Edward Huntington Williams

Professor J. J. Thompson believes that the source of energy is in the atom itself and not external to it. "The reason," he says, "which induces me to think that the source of the energy is in the atom of radium itself and not external to it is that the radio-activity of substances is in all cases in which we have been able to localize it a transient property. No substance goes on being radio-active very long. It may be asked, how can this statement be reconciled with the fact that thorium and radium keep up their activity without any appreciable falling off with time. The answer to this is that, as Rutherford and Soddy have shown in the case of thorium, it is only an exceedingly small fraction of the mass which is at any one time radio-active, and that this radio-active portion loses its activity in a few hours, and has to be replaced by a fresh supply from the non-radio-active thorium."*11*

If Professor Thompson's view be correct, the amount of potential energy inherent in the atom must be enormous.

RADIO-ACTIVITY AND THE STRUCTURE OF THE ATOM

But whatever the source of the energy displayed by the radio-active substances, it is pretty generally agreed that the radio-activity of the radio-elements results in the disruption of their atoms. Since all substances appear to be radio-active in a greater or less degree, it would seem that, unless there be a very general distribution of radio-active atoms throughout all substances, all atoms must be undergoing disruption. Since the distribution of radio-active matter throughout the earth is so great, however, it is as yet impossible to determine whether this may not account for the radio-activity of all substances.

As we have just seen, recent evidence seems to point to the cause of the disruption of radio-active atoms as lying in the atoms themselves. This view is quite in accord with modern ideas of the instability of certain atoms. It has been suggested that some atoms may undergo a slower disintegration without necessarily throwing off part of their systems with great velocity. It is even possible that all matter may be undergoing transformation, this transformation tending to simplify and render more stable the constituents of the earth. The radio-active bodies, however, are the only ones that have afforded an opportunity for studying this transformation. In these the rapidity of the change would be directly proportionate to their radioactivity. Radium, according to the recent estimate of the Curies, would be disintegrating over a million times more rapidly than uranium. Since the amount of transformation occurring in radium in a year amounts to from 1-2000 to 1-10,000 of the total amount, the time required for the complete transformation of an atom of uranium would be somewhere between two billion and ten billion years—figures quite beyond the range of human comprehension.

Various hypotheses have been postulated to account for the instability of the atom. Perhaps the most thinkable of these to persons not specially trained in dealing with abstruse subjects is that of Professor Thompson. It has the additional merit, also, of coming from one of the best-known investigators in this particular field. According to this hypothesis the atom may be considered as a mass of positively and negatively charged particles, all in rapid motion, their mutual forces holding them in equilibrium. In case of a very complex structure of this kind it is possible to conceive of certain particles acquiring sufficient kinetic energy to be projected from the system. Or the constraining forces may be neutralized momentarily, so that the particle is thrown off at the same velocity that it had acquired at the instant it is released. The primary cause of this disintegration of the atom may be due to electro-magnetic radiation causing loss of energy of the atomic system.

Sir Oliver Lodge suggests that this instability of the atom may be the result of the atom's radiation of energy. "Lodge considered the simple case of a negatively charged electron revolving round an atom of mass relatively large but having an equal positive charge and held in equilibrium by electrical forces. This system will radiate energy, and since the radiation of energy is equivalent to motion in a resisting medium, the particle tends to move towards the centre and its speed consequently increases. The rate of radiation of energy will increase rapidly with the speed of the electron. When the speed of the electron becomes very nearly equal to the velocity of light, according to Lodge, the system is unstable. It has been shown that the apparent mass of an electron increases very rapidly as the speed of light is approached, and is theoretically infinite at the speed of light. There will be at this stage a sudden increase of the mass of the revolving atom, and, on the supposition that this stage can be reached, a consequent disturbance of the balance of forces holding the system together. Lodge considers it probable that under these conditions the parts of the system will break asunder and escape from the sphere of one another's influence.

"It is probable," adds Rutherford, "that the primary cause of the disintegration of the atom must be looked for in the 1 ss of energy of the atomic system due to electro-magnetic radiation."*12*

Several methods have been devised for testing the amount of heat given off by radium and its compounds, and for determining its actual rise in temperature above that of the surrounding atmosphere. One of these methods is to place some substance, such as barium chloride, in a calorimeter, noting at what point the mercury remains stationary. Radium is then introduced, whereupon the mercury in the tube gradually rises, falling again when the radium is removed. By careful tests it has been determined that a gram of radium emits about twenty-four hundred gram-calories in twenty-four hours. On this basis a gram of radium in a year emits enough energy to dissociate about two hundred and twenty-five grams of water.

What seems most remarkable about this constant emission of heat by the radium atom is that it does not apparently draw upon external sources for it, but maintains it by the internal energy of the atom itself. This latent energy must be enormous, but is only manifested when the atom is breaking up. In this process of disruption many of the particles are thrown off; but the greater part seem to be stopped in their flight in the radium itself, so that their energy of motion is manifested in the form of heat. Thus, if this explanation is correct, the temperature of the radium is maintained above that of surrounding substances by the bombardment of its own particles. Since the earth and the atmosphere contain appreciable quantities of radio-active matter, this must play a very important part in determining the temperature of the globe—so important a part, indeed, that all former estimates as to the probable length of time during which the earth and sun will continue to radiate heat are invalidated. Such estimates, for example, as that of Lord Kelvin as to the probable heat-giving life of the sun must now be multiplied from fifty to five hundred times.