STUDIES OF ATOMIC NUCLEI1

It is well known that a study of the single scattering of a particles by the elements led to the nuclear theory of the atom, and has provided us with a direct method of measuring the nuclear charge of the elements. The pioneer experiments in this field were made by Geiger and Marsden in 1913, who showed that the number of particles scattered at different angles was in close accord with the nuclear theory, assuming an inverse square law for the forces between the a particle and the nucleus. The variation of scattering with velocity of the a particle was in close accord with this law. Their results were subsequently extended by the experiments of Chadwick, who made direct measurements of the nuclear charge. Most of the experiments of Geiger and Marsden were made with silver and gold. Using a particles of average range about 4 cms of air, they found that the law of inverse square held, at any rate approximately, for the closest distance of approach of the a particle, viz., about 4× 10-12 cms for gold. These results suggested that the nuclei of even the heavy elements must be of radius less than this small distance.

In a collision of an a particle with a light atom the distance of approach in a close collision is much smaller than the above, and direct evidence has been obtained that the law of the inverse square breaks down completely in the case of a close collision between an a particle and a hydrogen nucleus. More recently Bieer compared the scattering of a particles by aluminium and gold, and found the relative number of particles scattered by aluminium and gold to decrease as the angle of scattering was increased. Assuming that the scattering by gold was normal— i.e., in agreement with an inverse square law-he suggested that the discrepancy in aluminium might be due to the combined action of an attractive force superimposed on a normal repulsive force. From calculation he concluded that an attractive force varying as r1 fitted in best with his experimental results. In the light of these conclusions it became of importance to re-examine the question whether the law of inverse square holds accurately for the heavy elements for the closest possible distances of approach, and to determine as accurately as possible the variation of the scattering with velocity for the lighter elements.

1 Abstract of a lecture given before the Royal Institution of Great Britain, March 27, 1925.

For this purpose, Dr. Chadwick and I have used a modified method, and examined for a number of elements the scattering at an angle of about 135°, for a particles of different velocities. The source of a rays (radium B and C deposited on a metal disc) was hermetically sealed in a brass tube covered at one end by a thin film of collodion of stopping power about 2 to 3 mm of air for a rays. This was necessary to avoid radioactive contamination. The a rays fell on the radiator of small area of stopping power 2 to 5 mms of air carried on a frame of Acheson graphite. Extra care was taken to avoid extraneous scattering. By introducing thin sheets of mica of known stopping power in the path of the a rays, the variation of the number of a particles with velocity was directly determined. In this way measurements were made for a particles between the ranges of 6.7 cms and 2 cms. The straggling of the a particles in their passage through the mica made this method unreliable for shorter ranges of the a particles.

In this way it was found that the number of particles scattered by silver and gold through an average angle of about 135° varied inversely as the square of the energy of the incident a particles over the whole range. In the case of gold measurements were also made for smaller angles by the direct method of measuring the nuclear charge, previously used by Chadwick, and no variation in the law of the inverse square could be observed. Similar results were observed with a thin film of uranium obtained on graphite by sputtering. In this case it was difficult to determine the actual thickness of the film, but the variation in number of scattered a particles with velocity followed the regular law.

In the case of aluminium an unexpected result was observed. The scattering for 6.7 cms a particle was less than the theoretical, and rapidly decreased as the velocity of the a particles was diminished, falling to a minimum for particles of range about 5 cms. It then increased again and tended towards the theoretical value for slow-speed a particles. It is of interest to note that the velocity of the a particles for this minimum of scattering is about the same as the minimum velocity required to liberate protons from aluminium. It should be mentioned that even for the swiftest a particle the number of protons of range greater than 3 cms which appeared at an angle of 135° was only a small fraction of the total number of scattered a particles, indicating that the expulsion of a proton does not occur in all close collisions of the a particle with the nucleus.

From the observations on the scattering by a film of uranium, it is clear that the law of inverse squares holds, at any rate approximately, up to the closest distances of approach-viz., about 3 × 10-12 cms. This

raises a difficulty, since from radio-active evidence it is believed that the nuclear structure of uranium extends to a distance of about 7 × 10-12 cms. If the a particle liberated in the disintegration of uranium gains most of its energy in escaping in the repulsive field of the nucleus, its position before disintegration can not be less than 7x 10-12 cms from the center of the nucleus, and may be greater if it is liberated with initial energy. It is for this reason that it has been considered probable for some time that the radioactive elements consist of a central charged nucleus surrounded at a distance by a satellite distribution of positively and negatively charged particles. If these form a symmetrical doublet-like structure extending over a short linear distance, it may be difficult to detect the presence of such a satellite distribution by direct scattering experiments. A similar idea may be used to explain the peculiar behavior of the aluminium nucleus. In such a case the distance apart of the satellites or doublets may be comparable with their distance from the main nucleus. It is easy to see on such an assumption that a particles of appropriate speed may just enter the satellite region where the electrical field may be relatively weak, and thus show a marked defect in scattering. Swifter particles pass through the satellite distribution and come under the strong electric field of the central nucleus, where they are more effectively scattered.

It is obvious that an accurate determination of the scattering of a particles by the elements may supply important evidence, not only on the size of the nucleus, but on some of the details of its structure. A large amount of accurate data will be required to test the adequacy of any theory of nuclear structure.

Since the proof that protons can be expelled from the nuclei of many light elements, the fate of the bombarding a particle after the disintegration has been a matter of conjecture. To throw light on this question Blackett has recently photographed by the well-known expansion method the tracks of more than 400,000 a particles in nitrogen. In addition to a number of branching tracks which obey the laws of a perfectly elastic collision, eight branching tracks were observed in which the laws of an elastic collision were not obeyed. In these photographs the fine track of the proton was clearly visible, and also that of the recoiling nucleus, but in no case was there any sign of a third branch due to the escaping a particle. He concluded that the a particle was captured in a collision which led to the ejection of a proton. The branches were co-planar, indicating that momentum was conserved in such collisions. The length of the tracks of the proton and recoiling nucleus was in good accord with such an assumption. These ex

MACKENZIE PARK AS A FIELD FOR SURVEY, EXPLORATION, LITERATURE AND ART

MACKENZIE PARK and the surrounding district affords a splendid opportunity for university summer parties, students, professors and others desiring a field in which to carry on surveys and explorations, writing and art work, for practice, experiment or other purposes. Mackenzie Park is the name locally applied in the Norway of Canada to a strip of country approximately twenty miles north and south by seventy miles east and west lying near the southern edge of the bottom lands of Bella Coola River and the eastern shore of South Bentinck Arm. Mackenzie Park is at the head of one of the longest fiords midway of the coast of British Columbia. It was so named in honor of Sir Alexander Mackenzie, the first white man to cross America north of Mexico and, who, surfeited with scenery in his long trip from Montreal through the Canadian Rockies, wrote superlatively of the scenery of the area now known as Mackenzie Park.

A petition has been made that this area might be turned over for administration by the Dominion Parks as a great out-of-doors museum and sanctuary for the conservation of animal and plant life, beautiful scenery and pure water. Any surveys and explorations, literature and art treating of the park or vicinity would be conducive to this end.

As a field for physiography, topography and mapping the area is excellent, being unsurveyed British Columbia Crown lands extending from sea level to about 10,000 feet altitude. It consequently affords ample opportunity for either practice or practical work. An aeroplane photographic survey would be useful in developing the park.

From the geographic and geological standpoint, many parts of the park have never been seen by white men. The many glaciers and waterfalls should prove of interest. From one point on Mackenzie Highway eighteen glaciers may be seen. Southeast and partly within the park is a glacier which is said

to be forty miles long. The great number of glaciers ensures that a variety of glacial problems may be presented. Two glaciers may be seen from the Bella Coola Post Office and another from a point one mile up the road. In the park are thousands of waterfalls, some of them large. One near the eastern edge of the park is said by reliable frontiersmen to make a clear leap of over 828 feet. If this be true, it is the fifth highest known fall in the world, second highest in the western hemisphere and the highest in Canada. Surely the opportunity to first measure this fall or to take good large photographs, motion pictures, sketches or paintings of it should appeal to many university students or men of leisure. Hot springs are found on South Bentinck Arm.

Great

From a botanical standpoint the park and surrounding region are practically unknown. variety of plant life and plant problems may be expected in this area, which extends from the sea, salt marshes and lowlands to the mountain peaks on the one hand and which presents climatic conditions ranging from the rather moist sea coast climate to the semi-arid conditions of the region embracing the eastern end of the Bella Coola valley where irrigation is practiced. Throughout the region there are four species of giant trees, thousands of them being over six feet in diameter-red cedar, cottonwood, Douglas fir and aeroplane spruce. On the western edge of the park the vegetation is luxuriant and semi-tropical. On the east are Jack-pine barrens.

Zoologically the area is interesting. Grizzly bear tracks may certainly be found within twenty-four hours after arrival at Bella Coola, at least in August or September. A hunter living near Mackenzie Highway in 1924 left home in the morning, went on foot to a glacier in the park not far distant and was able before supper time to bring back a mountain goat. Fourteen eagles have been counted on a single stub at the edge of the park area. The sea is very deep off Bella Coola and in it is such life as sea fans and sand sharks. The several varieties of salmon taken in North Bentinck Arm for the two canneries within five miles of Bella Coola offer many zoological problems.

From the anthropological standpoint the Bella Coola Indians are of interest. They are of the Salish linguistic stock and North Pacific Coast culture. They live within two miles of Bella Coola and may be seen working for the canneries. About one fifth of the known petroglyphs of Canada lie within a day's motorboat run of Bella Coola. The material culture, social organization and folk-lore of the Bella Coola has been studied for the National Museum of Canada, but much remains to be done. In archeology, linguistics and physical anthropology, the field is almost untouched. The rituals and dramas may still be seen

on occasion by the sympathetic, but they can not be easily seen by the typical tourist.

mer.

Carrier and Chilcotin Indians, who both belong to the Athapascan linguistic stock, visit Bella Coola and camp at several places in the valley during the sumEach group stays several weeks and some of them return a number of times in the season, producing a picturesque subject for observation and study. Much yet remains to be done on the ethnology, archeology, folk-lore, physical anthropology and linguistics of these people.

In 1895 a colony of about 200 people of Norwegian extraction settled in the Bella Coola valley. These fine, sturdy, hospitable Canadians still occasionally prepare Norwegian dishes, do Norwegian carving, painting and embroidery and a few still have examples of the old country costume and jewelry. Here is opportunity for the study of folk-lore, carving and painting.

To the literary man, the district presents many features of interest. There are two types of Indians, one a sea coast communal sedentary folk, the other horsemen of somewhat nomadic habits. There is all the charm of the Norwegian fishermen living on farms, and the loggers engaged in hand, horse and railroad logging. All the races meet in the salmon canneries with their cosmopolitan crews of Norwegians and Chinamen, Japs and Indians, to say nothing of the Scotch engineer, the French-Canadian and the "American." There is also the romance of the trappers and prospectors that has not yet been adequately told. The cowboy life that drifts down from the interior and may usually be seen in evidence at Bella Coola and the Indian horse-men and horse-women are perhaps less modernized than anywhere else in Canada.

All the subjects of such great interest to the student and literary man have an equal attraction for the artist, the movie operator and the artist-photographer. Within or adjacent to the park are strange and wonderful types of men, glaciers and waterfalls. Within the forests are vast green roofed "cathedrals," pillared with tree trunks, carpeted with moss and illuminated by sunlight strained through green leaves and lending a glamor to the scene that can not be simulated by the man-made stained glass.

This region is reached by a weekly steamer from Vancouver for Bella Coola at a cost of $20.20, one way, including meals and berth. The traveler usually leaves Vancouver Wednesday night and reaches Bella Coola sometime Sunday, and has a good opportunity to see intimately the natural resources, industries and life of the coast as the steamer stops at Indian villages, salmon canneries and logging camps, often for a considerable time. Wholesome meals may be had throughout the Bella Coola valley at fifty cents and bed at. the same rate.

The western edge of the park and part of the northern edge can be viewed from motor or sail boats on North and South Bentinck Arms. The next section of the northern edge of the park is adjacent to the automobile road, locally known as Mackenzie Highway, which now extends up Bella Coola valley for forty-one miles. Beyond this is a good pack trail for horses. Along the land route mentioned are post offices at Bella Coola, Hagensborg and Atnarko. There is telegraph communication with the outside world from Bella Coola and Atnarko. One may live in delightful simplicity and comfort at various places along the Bella Coola valley road with telephone and weekly mail service and yet walk in the park area to some glaciers and many waterfalls and return home in time for supper.

Here one may live in comfort among a picturesque and primitive pioneering people with unequalled scenery of mountains, 'glaciers and virgin forests at his very door. The person who longs for a complete change and yet hesitates to break connection with his business may enjoy all the wilderness that his heart calls for and yet be within telegraphic communication with the world of affairs. As a location for the summer field work conducted by many of the universities, Mackenzie Park is unequalled, since here is presented opportunity for study of several lines of natural history. Contributions to such study will be to the benefit of the park proper and to the country in general.

OTTAWA, CANADA

HARLAN I. SMITH

THE UNSATISFACTORY STATUS OF THE GLACIAL CONTROVERSY* INTEREST is revived in the problems of the cause or causes of "glacial periods" by the conclusions expressed in the well-written "Text-book of Geology," by Professors Pirsson and Schuchert.1

There are no more controverted and widely discussed problems in geology and terrestrial physics than those connected with the cause or causes of the glaciations, which, during short intervals throughout geologic time and at widely separated latitudes, have left their traces in and upon the crust of the earth. From the earliest Proterozoic to the present time tillites and tills mark glacial action.

In this latest authoritative text we find the following unsatisfactory conclusion, which fully expresses the general consensus of opinion on the subject, both as to the separate and combined causes.

*See also, "Ancient climates," Scientific Monthly, May, 1925, pp. 459-479.

1 1924, 2nd Edition, Jno. Wiley & Sons, Inc.

GLACIAL CLIMATES

As yet there is no accepted explanation of why the earth from time to time undergoes glacial climates, but it is becoming clearer that they are due rather to a combination of causes than to a single cause. Probably the greatest single factor is the high altitude of continents, with great chains of new mountains (the hypsometric causes) which disturb the general direction and constitution of the air currents (the atmospheric causes) and the ocean currents as well.2

It is permissible to state, however, that the glacial climates are irregular in their geologic appearance, are variable latitudinally, as seen in the geographic distribution of the tillites between the poles and the equatorial region, and finally that they appear in geologic time as if suddenly introduced.3

Now, there is no adequate evidence that the earth as a whole has ever gone through a glacial climate. Local and alpine glaciations have recorded themselves time and time again in all latitudes between polar and equatorial regions; and there have undoubtedly been two great glaciations, that of Permo-Carboniferous and that of Pleistocene time. These were widely apart in latitude, and neither encroached upon the glaciated latitudes of the other, although there was a notable lowering of the snow line in tropical and equatorial latitudes in the final chill of Pleistocene time.

These glaciations were separated in time by the vast interval between the Paleozoic and the dawn of the Postglacial or Human Epoch. The former, with its maxima localized between latitudes 15° and 35°, and the latter with its maxima in about latitudes 50° to 55°.4

These maxima approximately coincide respectively with the arid and the north and south temperate rain belts of to-day, the former at the beginning, and the latter at the close of the vast geologic interval above mentioned.

There is abundant evidence that during all these glaciations, except the last, the oceans remained genial, and in this last glaciation the chill was restricted to oceans in polar and mid-latitudes; equatorial and adjacent oceans remained genial or similar to the genial and mild ocean temperatures which were so widely distributed throughout geologic time.5

But locally and at widely separated latitudes temperature fluctuations upon continents were far different. "The hypsometric causes" probably operated 2 p. 660.

actively in most of the known instances of glaciation, but in the great zonal glaciations of the Permo-Carboniferous and of Pleistocene time these causes do not appear to have exercised any effective control, for the glacial flows seem to have overridden mountain ranges and extended to and, in some instances, into oceans remote from the centers of dispersion.

Realizing that the ratio of ocean areas to continental areas is and probably within geologic time has not been far from 3 to 1 and that the two great zonal glaciations were separately restricted to less than half the remaining continental areas, it is difficult to see how it can be said that "the earth from time to time undergoes glacial climates." It appears rather that glaciations, even in these great extensions, were local phenomena, which from time to time were imposed upon restricted portions of continents; and were imposed upon oceans but once, and were then restricted to polar and mid-latitude regions. There is, therefore, a manifest need, even in this latest text, for a thorough reconsideration of the subject, not only of the causes of glaciation, but of the long and widespread genial conditions which were interrupted by short local glaciations as recited in the text under consideration.

Referring to variation in solar radiation as a cause of glaciation, the authors say:

Moreover, Huntington states that five authorities on glaciation have concluded that if the mean temperature of the earth were to fall 9 degrees to 11 degrees F. and were to remain thus low for a sufficient length of time, meteorological conditions would be so altered that a large part of North America would be covered with ice down to about the fortieth degree of latitude and Europe. would suffer a corresponding glaciation.6

This is not borne out by existing facts; the mean temperature of North America and Europe in the latitudes named is about 40° F. A reduction of from 9 to 11 degrees F. would bring the mean temperature of these regions down to about 30 degrees F. Portions of North America, Europe and Asia are now at or below this mean annual temperature, and have been under these general meteorological conditions for a long period in the past, and are not glaciated. Furthermore, it is found by all observers that the mean annual temperatures of Antarctic and Arctic regions are below this temperature, and that these regions are being deglaciated."

It is difficult to accept deductions that North America and Europe down to 40° latitude would be reglaciated at a mean temperature higher than that now deglaciating Antarctica, unless the insolation 6 p. 663.

7 SCIENCE, N. S. Vol. XLVI. Dec. 28, 1917, pp. 639–640.