In his 1921 address to the British Association for the Advancement of Science, Lord Rayleigh said: “It appears certain that the radioactive materials present in the earth are generating at least as much heat as is now leaking out from the earth into space. If they are generating more than this (and there is evidence to suggest that they are), the temperature must, according to all received views, be rising.”
CHAPTER IV
AN EPOCH-MAKING DISCOVERY
When radium was discovered by Mme. Curie in 1898, the effect upon the scientific world was startling, not to say “catastrophic”—as one author wrote at the time—since its activities ran counter to every known principle of physical science. “Some of the most solid foundations of science were destroyed, some of its noblest edifices wrecked, and scientists had to nerve themselves to face and investigate a new form of energy.”
So soon as radium compounds (salts) became available, however, the amount of energy given out in radioactive processes—the emission of powerful radiations which can be transformed into light and heat—was measured; and it was found that radium, weight for weight, gives out as much heat as any known fuel every three days, and in the course of fifteen years releases a quantity of energy nearly 2,000 times as much as is obtained from the best fuel, with no signs of exhaustion (Soddy). In the combustion of coal, the heat evolved is sufficient to raise a weight of water some 80 to 100 times the weight of the fuel from the freezing-point to the boiling-point. The spontaneous heat from radium is sufficient to heat a quantity of water equal to the weight of radium from the freezing-point to the boiling-point every three-quarters of an hour. In other words, a pound of radium contains and evolves in its changes the same amount of energy as 100 tons or more of coal evolve in their combustion.
In ordinary chemical changes it is the molecules (groups of atoms) which are altered or rearranged; in radioactive change the atoms themselves suffer disintegration and rearrangements. The energy of radioactivity, then, is—according to the accepted view—intra-atomic—stored-up energy within the atom itself. It was calculated by Prof. Curie that the energy of one gram of radium would suffice to lift a weight of 500 tons to a height of one mile. If it were possible to obtain one cubic centimeter (a thimbleful) of the “emanation” from radium in the form of a gas, we should find that it possessed the power, altogether, of emitting more than seven million calories of heat! A thimbleful of this invisible gas would be more than sufficient to raise 15,000 pounds of water 1°. But in every mass of radium, small or large, not more than 13 trillionths of it is undergoing change per second.
“The processes occurring in the radio-elements,” says Rutherford again, “are of a character quite distinct from any previously observed in chemistry. Although it has been shown that the radioactivity is due to the spontaneous and continuous production of new types of active matter, the laws which control this production are different from the laws of ordinary chemical reactions. It has not been found possible in any way to alter either the rate at which the matter is produced or its rate of change when produced. Temperature, which is such an important factor in altering the rate of chemical reactions, is, in these cases, entirely without influence. In addition, no ordinary chemical change is known which is accompanied by the expulsion of charged atoms with great velocity.... Besides their high atomic weights, [they] do not possess in common any special chemical characteristics which differentiate them from the other elements.”
It was early observed by Curie and Laborde that the temperature of a radium salt is always a degree or two above that of the atmosphere, and they estimated that a gram of pure radium would emit about 100 gram-calories per hour. Giesel later showed that radium was always at a temperature 5° higher than the surrounding air, regardless of what the temperature of the air might be. This continues unchanged whether the temperature of the surroundings be 250° below zero Centigrade, or in the intense heat of an electric furnace.
“Perhaps,” remarks a writer in The Scientific American (February, 1922), “there will come a time when we shall use the energy in the atoms to drive our machines, cook our food and heat our rooms. Besides, already today we are actually using—even if only a very tiny part—the atomic energy. Thus, for instance, the rays emanating from radium are used for therapeutic purposes and the electrons emanating from a glowing filament can be directed so easily that they can be used in a large number of apparatus for wireless telegraphy and telephony. Most probably plants also make use of this energy in their growth because it has been demonstrated that the rays of the sun liberate electrons from the green leaves, and lastly it may also be mentioned that we humans use a little of this intra-atomic energy when seeing with our eyes, which we are enabled to do by the photoelectric action of light.”[A]
During the course of the process of disintegration, atoms of uranium and thorium and their products give rise to no fewer than 36 different substances (A. S. Russell), and of these at least a dozen are “new elements.”
All of the 36 radioactive elements are disintegration products of one or the other of the two parent elements, uranium and thorium. They are arranged by the chemist in three series: namely, Uranium 1, Uranium 2 (the Actinium Series), and Thorium. In the first series there are known to be 15 transmutations of matter; in the second, 11; and in the third, 10. The periods of “half change”—the period required for one-half of a given quantity of a radioactive element to decompose—of the different radioactive elements vary all the way from thousands of millions of years for the longest lived primary elements—2.6x1010 years for thorium, 8x109 for uranium 1—to .002 second for actinium A. In the case of radium itself, 1,670 years are demanded for the disintegration of half of any portion, according to the exact measurements of Profs. B. O. Boltwood and Ellen Gleditsch. The stable end product appears to be in each case an isotope of lead—leads having similar chemical properties but of different atomic weights (i.e., different atomic composition).