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up into drops so small that they could just be seen with the highest power of the microscope, scattered evenly through the water in a dense cloud. This mercury-water emulsion was as black as ink. Professor Wood believes that a possible future application of this newly discovered power of these waves may be to create emulsions out of combinations of liquids that can not be forced to mix in any other way.

RADIO MEASUREMENTS

RADIO measurements are now made a thousand times as accurately as four years ago. Where in 1923 radio engineers made their measurements to an accuracy of 1 per cent., they now go to a thousandth of a per cent., according to a paper given by Dr. J. H. Dellinger, in charge of the radio laboratory of the U. S. Bureau of Standards, at the meeting of the American section of the International Union of Scientific Radio Telegraphy.

Part of this increased accuracy has been due to the use of the piezo-electric oscillator. With this device, a small crystal of quartz, between two metallic electrodes, can control the wave length of a broadcasting station to a considerable degree of accuracy. However, Dr. Dellinger stated that the use of the crystal by no means dispenses with careful adjustments. By the use of the crystal oscillator it is easily possible to obtain an accuracy of a tenth of a per cent., but for a higher degree of accuracy all the conditions, such as the temperature of the crystal, must be carefully controlled.

If American and Canadian broadcasting stations do not keep to their proper wavelengths, it will not be for the lack of accurate standards, for Dr. Dellinger told of comparisons made by the Bureau of Standards with the Canadian radio authorities. One of the bureau's standard crystal oscillators has been sent to Canada for comparison with their standards, and it has been found that the two agree to within a hundredth of one per cent.

M. S. Strock, also of the bureau, told of its work in disseminating standard frequencies of radio waves. One way of doing this is by calibrating meters sent in by outside agencies with the bureau's standards, but the most effective way is by the use of the transmitting stations themselves. "The basis of this scheme," he said, "depends upon the fact that a standard of radio frequency may, neglecting the effects of interference, be transmitted over great distances and reproduced at the receiving station with an accuracy equal to that of the transmitter.''

This method is used by sending out regularly standard frequency signals from the Bureau's station WWV. Careful check is kept also of the frequency of a selected list of stations near enough to Washington to be received directly at the bureau. There are thirteen of these standard frequency stations, and in addition, a list of "constant frequency" stations, which includes about 5 per cent. of the stations of the country, are checked a little less carefully. Standard frequency signals have been broadcast from 6XBM, at Stanford University, Californio; 1XM, at the Massachusetts Institute of Technology, and 9XL, of the Gold Medal Flour Company, near Minneapolis.

FOSSIL-BEARING ROCKS OF THE GRAND CANYON

THE fossil-bearing rocks of the Grand Canyon, which have recently aroused much interest because of the discovery in them of footprints of long-extinct animals, are now yielding remains of the leaves and stems of plants among which these animals roamed and fed, many millions of years ago. At the meeting of the National Academy of Sciences, Dr. David White, of the U. S. Geological Survey, told of his examination and identification of many specimens from this region.

The plants that grew on the ancient floodplain of red sand through which the great gash of the Grand Canyon has since been cut were very little like the ones that grow in the forests of to-day. Their nearest relatives still living are the ferns and tropical cycads and similar plants.

The plant remains were all preserved by being deposited at the bottoms of streams or ponds, but there is evidence that these bodies of water were not permanent, but appeared during rainy seasons and dried up when the rains ceased, just as most of the streams and ponds in the arid Southwest do to this day. Pressed between thin layers of water-deposited sand that gradually hardened into strata of sandstone, the leaf and stem fragments were preserved like botanical specimens in a huge book, and even after their own substance had gradually decayed and almost wholly vanished, their imprints were left on the stone just as the print of a pressed flower is sometimes left between the pages of an old volume, shelved many years ago and never opened.

The knowledge gained of the ancient animals of the Grand Canyon region was summarized and brought down to date by Dr. Charles W. Gilmore, of the U. S. National Museum. Footprints of extinct reptiles and amphibians have been found in three distinct formations. These are known respectively as the Coconino, Hermit and Supai formations. Thirty-three species were represented by tracks in these three levels, 22 species in the Coconino, 8 in the Hermit formation, which also yielded all the plant fossils studied by Dr. White, and 3 species of animals in the Supai. Remains of insects were found for the first time in the Hermit formation. More than two tons of stone slabs bearing fossil footprints are now in the U. S. National Museum.

STEAM WELLS

BORING for live steam as men bore for oil, with the possibilities of running engines and turning dynamos without the burning of a pound of fuel, was described before the meeting of the National Academy of Sciences by Dr. Arthur L. Day, of the Carnegie Institution of Washington. The steam wells described by Dr. Day are in Sonoma County, Calif., where operations have been going on for some time to exploit a field of hot springs and steam vents similar to those of Yellowstone National Park, but on a smaller scale.

So far five borings have been sunk. They reach depths of from 300 to 600 feet, yielding a total of nearly 5,000 horsepower of live steam. The temperatures at the bottoms of the wells vary from 160 to 185 degrees, and the

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pressures attain a maximum of 276 pounds per square inch. Besides steam, various gases come out of the wells, making up less than two per cent. by volume of the product.

Similar wells have been operated on a large scale for several years in Italy. Concerning these, Dr. Day remarked, "Compared with the development of natural steam in Tuscany, where more than 30,000 H.P. is now commercially developed, the conditions in California appear to be somewhat more favorable from the point of view of the uncondensable gases carried and their corrosive effect upon metals. The total power available is probably smaller. The oldest of the California wells has now been flowing intermittently for five years with undiminished pressure."

VANADIUM

THE addition of a new metal, vanadium, to the world's resources, is announced by J. W. Marden and M. N. Rich, research scientists of the Westinghouse Lamp Company.

Vanadium has been known in its compounds for a long time, according to Dr. Marden and Dr. Rich, but in spite of a century of efforts on the part of chemists no one has previously been able to produce it in its pure form. The method employed by the authors is to heat a mixture of vanadic oxide, metallic calcium and calcium chloride in an electric furnace for an hour at a temperature of nearly 1,400 degrees Fahrenheit. After cooling and stirring the resulting mass in cold water, metallic vanadium is obtained in the form of beads.

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"The beads of vanadium are very bright, have a steelwhite color and are quite malleable, soft and ductile,'' say the authors. They can be melted in a vacuum in a high-frequency induction furnace, rolled into wire and worked up into other shapes. As far as analysis can determine, they are 99.9 per cent. pure metal.

"There is no known use for this new metal at present, but undoubtedly it will have special properties that will make it useful. Tungsten, for example, was once a useless metal, but is now of inestimable value for filaments in incandescent lamps, for high-speed tool steel alloys and many other purposes. Vanadium may, in time, prove equally serviceable.''

ITEMS

USING an electrical heat-measuring device so incredibly delicate that it is sensitive to two trillionths of an ampere of current and will measure temperature changes of as little as one ten millionth of a degree Centigrade, Dr. A. V. Hill, of Cornell University, has measured the temperature changes in nerve fibers during their activity. In describing his experiments before the National Academy of Sciences, he stated that his object had been to learn more about the nature of nervous action. Older theories have held that nervous impulses were not like other physiological processes, but were physical waves like light or radio waves. These ideas were based on the absence of any detectable heat given off by nerves as a result of stimulation. But with the extremely sensitive instrument devised by Dr. Hill it is possible to measure

the almost vanishingly minute temperature rise that occurs in a single nerve fiber when it is caused to react. The moment of activity of a nerve is followed by a prolonged period of recovery, during which nine times the initial amount of heat is given off.

THE loss in weight that we all undergo every day has been the object of research by Dr. Francis G. Benedict and Cornelia Gollay Benedict, of the Carnegie Institution of Washington, who reported their results at the recent meeting of the National Academy of Sciences. Two sensitive balances were used in the work. Both were strong enough to sustain the weight of a man, but sensitive enough to register small changes in weight. One of the balances would indicate a change of one third of an ounce, and was so constructed that the volunteer for the experiment could sleep all night on its platform. The other was a hundred times as sensitive, but could be occupied for only an hour or so at a stretch. The total moisture losses of this class from a woman of average weight were found to average around 30 grams, or one ounce, per hour; for a man the figure was about one third higher. An auxiliary device permitted the separate measurement of losses from the lungs and skin, and these averaged 50 per cent. from each source of water loss. Other mechanisms measured the carbon dioxid given off, the percentage of water in the outgoing breath and also its temperature.

A NEW process for making sugar out of sawdust was described by Professor Erik Haegglund, of Abo, at the coal and wood chemistry conference recently held in Stockholm, where he reported that at Geneva, where the process is being tried on a commercial scale, from 65 to 70 per cent. of the sawdust by weight can be converted into sugar. For Sweden, where forestry is one of the dominating industries and where most of the sugar has to be imported, the method is likely to become of the greatest importance. Several hundred thousand tons of "wood sugar'' can be produced annually from easily accessible raw materials.

BECAUSE it resembles a star twinkling at night more than any other precious stone, the blue zircon of Siam has been rechristened "Starlite" by Dr. George F. Kunz. These flashing blue stones from Siam are unusual in that they are never blue at all when found in their natural state, but are brown or reddish in hue. The stones are placed in a crucible and burned in a fire from six to eight hours exposed to a solution of cobalt nitrate and potassium ferrocyanide, said Dr. Kunz, in announcing the new name to the New York Mineralogical Society. The chemicals do not touch the stones which are being changed in color, but the fumes do the coloring. After cutting it is necessary to expose them again to the fire from five to twenty minutes. This is the most brilliant blue and green precious stone. The flash is more near that of the diamond of the same color and resembles the occasional sparks from copper contacts, especially when a trolley pole hits a wire. It also has the brilliancy and color of some stars.

SCIENCE

VOL. LXV

MAY 6, 1927

No. 1688

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THE SOURCE OF SOLAR ENERGY1

INTRODUCTION

IT has been wisely said by Dr. W. W. Campbell that a scientist does not create the truth. He does nothing whatever to the truth; he simply uncovers it. Through the analysis of physical science, the universe is resolved into atoms-protons and electrons, and the cosmic laws are reduced to action and reaction of these integral parts. A general simplification has resulted; in the terms of atoms many complicated phenomena have been solved, and it is hoped that the new physics will shed some light on the problem in handthe source of solar energy.

It is known that throughout entire geological time the sun has been radiating energy at a rate which has varied but little. With the generally accepted estimate of the age of the earth2 each gram of the sun has accounted for about 2×109 calories, and the well-known problem arises: whence came this heat. The great quantity of the solar radiation and the inadequacy of the simpler theories to account for it have been so frequently discussed that a short review of them will suffice here.

It does not come within the scope of this paper to reexamine the data for determining the age of the earth. Estimates have ranged from 108 years to Russell's absolute maximum of 6 × 10o. Since even the minimum value given above is far in excess of that demanded by the following theories it is not necessary for our present purpose to defend any specific value. For the sake of definiteness we adopt the value 10o as of the proper order of magnitude, especially since this figure has apparently met with wider acceptance than any other.

(1) ORIGINAL HEAT

The sun radiates about two ergs per second, or 1.5 calories per year, for each gram of its mass. The researches of Emden, Eddington, Jeans and others have shown that, in order to maintain the observed mean density of 1.4 against the enormous pressures existing in the far interior, a critical temperature of some 10,000,000° to 30,000,000° K is required. The opacity of the interior, by setting up a negative temperature gradient, reduces the temperature of the photospheric surface approximately to 6,000° K.

1 Awarded the A. Cressy Morrison Prize in 1926 by the New York Academy of Sciences.

2 109 years.

The mechanism of the opacity is generally recognized to be the interaction of radiation and electrons with positive nuclei and highly ionized atoms in the center of the sun.

In spite of the great central temperature, it is immediately obvious that mere cooling is insufficient to account for the supply of the sun's heat. At the observed rate of radiation, and even assuming a maximum value for the specific heat, the lapse of a comparatively short period of time, say a million years, would certainly reduce the temperature so that the surface would be markedly cooler. The customary argument used in this connection is not valid. It has been said that since the average temperature of the sun falls at least one degree a year the surface would be sensibly cooler in even a few thousand years. The reduction of the central temperature by so small a fraction would scarcely disturb the temperature gradient. As far as historical time is concerned, the sun may well have been a hot body cooling according to the well-established laws of heat conduction. when we take geological time into consideration the case is different. We must find some source of heat to augment the original supply. We shall consider in turn the various possible ways in which the energy may have originated.

(2) CHEMICAL

But

Under this heading are grouped the processes which deal with the liberation of heat by the chemical combination of two or more atoms. Combustion is a specific example, governed by the equation:

C+20= CO2 + 2140 cal. per gm.

of reacting substance,

(1) which is equivalent to saying that the energy in one gram of uncombined carbon and oxygen, mixed in the proportion of one atom of the former to two of the latter, exceeds that in one gram of carbon dioxide by 2,140 calories. In no case has the heat of any chemical reaction been found to exceed ten times the value in the above equation. In the past billion years each gram of the sun will have radiated 105 times as much heat as could possibly be generated chemically. It is immediately obvious that chemical activity contributes practically nothing to the total energy of

the sun.

(3) GRAVITATION

(a) Meteoric. The discovery, by Rumford (1798) and Davy (1799), that heat has its mechanical equivalent in work, led to other theories regarding the source of solar energy. The first of these was Mayer's hypothesis that the radiation might be continually

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or about 20,000 times the heat produced by the complete combustion of a gram of carbon and oxygen, as shown above. The total annual radiation of about 3 × 1033 calories would, then, be equivalent to an influx of 8×1025 grams of meteors per year or 3.7 grams per square centimeter of surface per day.

Objections to this theory are many and serious. In the first place the density of meteoric matter in space, calculated from the frequency with which meteors are observed to strike the earth, is far too low to furnish even a fraction of the material required by the hypothesis. Furthermore, the increase of solar mass by such a process of accretion would produce corresponding accelerations in the movements of the planets, which could hardly have escaped detection and, finally, it is obvious that the heat produced by impact of solid matter on the solar surface would have a negligible influence on the steep internal temperature gradient. If, by any chance, the sun should happen to encounter during a given year the quantity of meteoric matter mentioned above, the life of the sun would not be lengthened in the least. Instead we would receive, during that year, twice the quantity of heat generally radiated by the sun.

(b) Contraction. Of all the theories of the origin of the solar heat, the one which has played the most prominent rôle is that put forth by Helmholtz in 1854. It, too, appeals to gravitation as a source of energy, but instead of the impact of exterior particles, it assumes a general contraction for the sun as a whole to renew the kinetic energy of the solar atoms. It is easily computed that a contraction of but one twentieth in the diameter of the sun per million years would generate enough heat to replenish that lost by radiation. Calculating backwards, we find that if the sun were originally an extended nebula, the average energy produced by contraction to its present state would be 27,000,000 calories per gram. Or, postulating the present rate of radiation as extending uniformly into the past, the minimum age we can derive for the sun, corresponding to an infinite initial radius, would be 18,000,000 years. This would be further reduced by considerations of the greater central density

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