A RADIUM CLOCK

A very interesting instrument was devised by Sir William Strutt (now Lord Rayleigh) which has been called a “radium clock.” It consists of a glass vessel containing a tube of radium salts in the center, from which two gold leaves are hung. The inner surface of the containing vessel is coated with tinfoil, and this foil is grounded. The radium salts cause the leaves to become electrically charged. They then diverge, and, coming in contact with the grounded tinfoil coating, they are discharged, only to fall back again and repeat the process. This clock will operate as long as the supply of radioactive material will act, which in the case of pure radium would be nearly 2000 years.

G. Lentner has recently succeeded in utilizing atmospheric potential by the aid of radioactive substances, which, in some way not yet clearly understood, exert an influence upon the transformer. The method is as follows: A post about 12 m. in height, forming a sort of antenna, is erected; the post ends in a collector consisting of an aluminum sphere provided with points covered with radioactive substances. This collector communicates by a conducting wire with a special transformer. Under these conditions the earth and atmospheric currents attract each other through reciprocal induction.

Dr. S. A. Sochocky, the well known radium expert, has made radium oil paints, and made paintings with them. “Pictures painted with radium look like any other pictures in the daytime, but at night they illuminate themselves and create an interesting and weirdly artistic effect. This paint would be particularly adaptable for pictures of moonlight or winter scenes, and I have no doubt that some day a fine artist will make a name for himself and greatly interest us by painting pictures which will be unique, and particularly beautiful at night in a dark or semi-darkened room.”

Dr. Sochocky also predicts that “the time will doubtless come when you will have in your own home (or someone you know will have) a room lighted entirely by radium. It would be possible today to illuminate a room, so that at night, without the aid of electricity or other artificial illumination, you could read fine newspaper print without difficulty. The light in such a room, thrown off by radium paint on walls and ceiling, would in color and tone be like soft moonlight, blue with a tint of yellow. Today, a room ten by nine feet could be illuminated in this way at a cost of $400, and the illumination would last ten years.

“However, such illumination will soon be much cheaper, because of new discoveries as to the best materials to combine with radium to produce light.”

CHAPTER III
RADIUM AND THE AGE OF THE EARTH

One of the important consequences of the discovery of radioactivity was to afford the scientist a means for solving the problem of the earth’s age. By “age of the earth” we mean here the time which has elapsed since the earth’s surface became fitted for the habitation of living beings. By means of radioactivity we can form an approximate estimate of the time which has passed since the formation of any given series of geological strata. Radium is our geological time clock.

It is now known that all the common rocks and soils of which the earth’s crust is built up contain measurable amounts of radium. According to the computation made by Prof. John Joly, the total quantity of radioactive matter may be as much as one 500 billionth part of the whole volume of the globe, or something over half a cubic mile.

All of the 36 known radio-elements are disintegration products of the primary radio-elements uranium and thorium—i.e., they are produced from one or the other of these in their long sequence of changes. And the rate at which the radioactive products change—their average life period,—from the first transmutation to the final product, radium lead, an isotrope of common lead, is accurately known. (Helium atoms are “the debris shed at the various stages of the transformation.”)

It is now well established that a gram of uranium as found along with its products in rocks and minerals is changing at a rate represented by the production of 1.88 x 10-11 grams of helium and 1.22 x 10-10 grams of lead (isotrope) per annum. We do not know for a certainty, of course, that this rate of production has been maintained throughout geological time. In the opinion of Lord Rayleigh, we may safely assume that the rate of transformation has not changed, so that “it would seem that in the disintegration of a gram of uranium we have a process the rate of which can be relied upon to have been the same in the past as we now observe it to be” (Nature, October 27, 1921).

Acting on Rutherford’s suggestion, the Hon. R. J. Strutt (later Lord Rayleigh) made a determination of the amount of radium in the superficial parts of the earth—which are alone accessible; and he also determined the ratio of the lead (isotope) to the uranium, which was found to be 1.3 (specifically, in the broggerite found in the Pre-Cambrian rocks at Moss, Norway). Now, if we assume—as the evidence seems to warrant—that the lead of this atomic weight (206.06) was all produced by uranium at the rate given above, we get an age of 925 million years for these rocks. Some minerals from other Archaean rocks in Norway give a rather larger figure.

“In other cases,” says Lord Rayleigh, “there is some complication, owing to the fact that thorium is associated with uranium in the mineral and that it, too, produces helium and an isotrope of lead of atomic weight probably 208 exactly, about one unit higher than common lead.”

Sir Ernest Rutherford estimated the time required for the accumulation of the radium content of a uranium mineral in the Glastonbury granitic gneiss of the early Cambrian as no less than 500,000,000 years. Later investigations give some of the Pre-Cambrian rocks an antiquity of 1,640 millions of years! The zoologist may now have all the time he wants for the slowly evolving organisms revealed by the sedimentary strata.

Prof. John W. Gruner, of the geology department of the University of Minnesota, discovered (in 1925) microscopic forms of plant life (algae) embedded in iron formations of the Vermillion Range near Lake Armstrong, Minnesota. Most of Minnesota’s iron deposits are due to the algae, Dr. Gruner thinks. The growth has the property of extracting iron from sea water and making of it a solid shell with which to surround itself. Accumulations of these iron shells through millions of years have been embedded in rock formations forming the iron ore.

Slices of rock a thousandth of an inch thick were examined under microscopes in the search for the algae. Algae began to flourish immediately after the earth, in cooling (according to one cosmological theory), got below the boiling point. Their form is much like seaweed, and they thrive at a temperature of 95° C. Dr. Gruner estimates the age of these algae-bearing deposits at 200,000,000 years, ten million years earlier than previous evidence showed.

If we employ the radioactivity test as a measure of geological time, the age of these fossil algae would have to be placed much higher—older by hundreds of millions of years. And the same must be said of the amphibian footprints recently (1925) discovered in the sandstone slabs of the Grand Canyon, by the caretaker on Hermit’s Trail, a thousand feet below the rim of the canyon. On the older geological time scale, these deposits date back some 50,000,000 years (lower Carboniferous period—the so-called “Mississippian” system). On the radium time schedule, these figures would need to be multiplied considerably (according to Boltwood and Holmes, by a multiple of six or more). It should be said, however, that on the time deposits of Walcott and Schuchert, based on the rate of deposition of sediments, the lower Carboniferous (Mississippian) deposits are not older than some 18,000,000 years.

But amphibian footprints are known from the far older Devonian period, whose strata are, on the radium basis, some 370 million years old.

Prof. Charles Schuchert, of Yale, regards the estimates of geological time based upon the rate of disintegration of radioactive minerals as, on the whole, far more reliable than estimates based upon the rate of deposition of sediments. No scientist pretends to be able to state exactly the age of strata by the amount of radium lead contained in them.

“In a third class of cases,” Lord Rayleigh points out, “the uranium mineral, pitchblende, occurs in a metalliferous vein, and the lead isotope produced in the mineral is diluted with common lead which entered into its original composition, ... but the complications cannot, I think, be considered to modify the broad result.

“A determination of the amount of helium in minerals gives an alternative method of estimating geological age; but helium, unlike lead, is liable to leak away, hence the estimate gives a minimum only. I have found in this way ages which, speaking generally, are about one-third of the values which estimates of lead have given, and are, therefore, generally confirmatory, having regard to leakage of helium.”

Dr. Homer P. Little, of the National Research Council, Washington, D. C., tells us (Scientific American Monthly, August, 1921, p. 173) that “from both calculation and experiment it is found that one gram of uranium will produce helium at the rate of one cubic centimeter in 9,600,000 years. The ratio between the amount of radium in a mineral and the amount of helium present therefore allows us to calculate the age of the mineral. The amount of uranium originally present compared to that left does not enter into the problem unless extreme lengths of time are under consideration, because of the fact that it is calculated to take 5,000 million years for one-half a given volume of uranium to disintegrate.

“It is perfectly true that much of the helium generated may escape. The assumption is, however, that in some minerals comparatively little escapes: zircon, particularly, seems to be an effective retainer. This mineral shows very effectively the increasing ratio of helium to uranium as consecutively older rocks are examined. Recent or Pleistocene specimens from Vesuvius show an apparent age of 1 million years; Miocene specimens from the Auvergne, France, of 6.3 million. The Devonian of Norway furnishes specimens 54 million years in age, and the Upper Cambrian of Colorado specimens of 141 million years; the Archaean of Ceylon, of the diamond-bearing rocks of South Africa, and of certain rocks of Ontario furnish specimens aged 286, 321 and 715 million years, respectively.”

The following table gives the mean of the results of Professors Boltwood and Holmes’ careful studies, based upon the accumulation of lead as a final product of the uranium series:

MILLIONS OF YEARS
Carboniferous340
Devonian370
Pre-Carboniferous410
Silurian or Ordovician430
Pre-Cambrian:
Sweden1,025
United States of America1,310–1,435
Ceylon1,640

These results, a total of 1,400,000,000 years, greatly transcend Lord Rayleigh’s (Strutt’s) earlier calculations regarding the antiquity they assign to Paleozoic and Pre-Cambrian times.

In 1918, Prof. Joseph Barrell reviewed the various methods employed and the results obtained in the attempt to determine from geological, chemical and physical evidences the time that has elapsed since the beginning of the Cambrian Period (when abundant fossil invertebrates are first met with), and reached the following time estimates for the principal divisions of the geologic record (exclusive of the Pre-Cambrian rocks):

The time thus established covers a period of from 550,000,000 to 700,000,000 years, or from ten to 15 times longer than has usually been accepted by geologists. Pre-Cambrian time was found to have a similar order of magnitude; but here the evidence rests largely upon the radioactivity of the crystalline rocks formed during this vast period.

It is now universally accepted that the time required for the formation of the Pre-Cambrian rocks was fully as long as, if not longer than, that for the succeeding geological divisions. The Archaean deposits have a vertical thickness, in the regions north of the Great Lakes, estimated at about 65,000 feet, or 12 miles. Their base, as a matter of fact, has never been reached. It is interesting to note that the granites of Norway, Canada, Texas and East Africa have an indicated age of 1,120,000,000 years, measured in terms of radium products. Prof. Henry Norris Russell, of Princeton University, concludes, from his careful investigations in radioactivity, that the age of the earth is “a moderate multiple of 1000 million years.”

Professor Joly has computed that if there are two parts of radioactive material for every million million parts of other matter throughout the whole volume of the earth, and this is considerably less than he has found on the average in the earth’s crust, then this earth, instead of cooling off, is actually now heating up, so that in a hundred million years the temperature of the core will have risen through 1,800 degrees centigrade.

Dr. Millikan observes (Science, July 9, 1921) that this is a temperature “which will melt almost all of our ordinary substances.... It means that a planet that seems to be dead, as this our earth seems to be, may, a few eons hence, be a luminous body, and that it may go through periods of expansion when it radiates enormously, and then of contraction when it becomes like our present earth, a body which is a heat insulator and holds in its interior the energy given off by radioactive processes, until another period of luminosity ensues.”

Lord Rayleigh’s series of researches for the purpose of determining the quantity of radium present in a number of representative rocks, both igneous and sedimentary, seems to prove that the average amount of radium in the earth’s crust is about 20 times larger than the amount calculated by Rutherford to be necessary to retain its temperature unaltered. Joly’s investigations revealed values in general agreement with these, but in many cases he obtained a value several times greater than the amount found by Lord Rayleigh. Further investigations showed that thorium is as widely distributed as radium in the earth’s crust, which is true also of uranium.

“Incredible as it may appear,” remarks Rutherford, “the radioactive bodies must have been steadily radiating energy since the time of their formation in the earth’s crust. While the activity of uranium itself must decrease with the lapse of time, the variation is so slow that an interval measured by millions of years would be required to show any detectible change.”

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).

[A] See Shipley, Maynard, “Electricity and Life,” ch. vi., Little Blue Book No. 722.

Isotopes are groups of elements which cannot be distinguished (or separated from) one another by any known chemical methods, and which differ only in the atomic weights of the members of the group. In the radioactive groups, the various elements differ also in degree of stability of their atoms.

Chemists cannot actually weigh the mass of an atom of an element on a pair of scales, or by any other method. But if we put down 16 as the “atomic weight” of oxygen, and ascertain the “combining weight” (ratio) of hydrogen to oxygen, we can determine the “atomic weight” of hydrogen (1.008). (See Shipley, “The A B C of the Electron Theory of Matter,” p. 14, Little Blue Book, No. 603.) The ratio of the masses of any two elements in a chemical compound can be very accurately determined. Without going into the details here, it may be said that the relative weights of the atoms of any element can be determined to 0.01% in many cases (by chemical analysis and synthesis); while the actual weight of any atom has not yet been determined to better than 0.1%.