the right hand disc to be fastened in different positions by the pin P. This arrangement allows objects of considerable width to be held by their edges. A supporting screw in the movable disc opposite the groove is sometimes used to prevent tipping and wedging of the disc under strong pressure, but it is generally not required.

Universal Tripod Bases.-Of all laboratory implements the tripod stand is probably the one that is most constantly and universally employed. A tripod base forms the foundation of a great number of scientific instruments; it is therefore desirable to have a number of accurately made bases for use with interchangeable apparatus, and adaptable to a great variety of purposes.

The ordinary way of fastening the standard to a tripod base is by means of a screw on the end of the rod. This is permissible when the rod is to be left in position permanently, but when it has to be removed frequently, it is very inconvenient, as a special wrench is required for the operation. When accurate construction is required it is necessary to reduce the diameter of the lower end of the standard to form a shoulder, and to cut the screw in a lathe. This adds considerably to the expense and difficulty of fitting apparatus to the base. A much simpler and better mode of attachment can be employed which has proved itself to be very satisfactory in my laboratory. The bases that I have made are of two sizes. The larger one covers a circle of 30 cm. and weighs 5 kilograms. Its center is a cylinder 8 cm. in diameter and 7 cm. in height. This base is like that of the large adjustable stand which I have described, and is turned and bored on the lathe in exactly the same manner as that. The central hole has a diameter of 19 mm. and the standards are clamped in it by a large brass screw which passes horizontally through the center of the hub. As the screw has a large head with four spokes like the hand-nut on the universal clamp figured in the drawing, the rods and bushings are held with the greatest firmness, but they can be changed almost instantly. This kind of attachment allows a

certain amount of vertical movement of the standard of a table, or of apparatus, when variation of height is desirable. When a more extensive elevation is necessary the tripod can be placed over a hole in the table through which the rod can pass, or it can be supported on rods clamped by brass set-screws in 13 mm. holes in the cylindrical feet. These supplementary rods may be used as substitutes for leveling-screws. If such screws are required they are made with brass cylindrical nuts which are clamped in the holes in the feet. These holes are exactly at right angles to the plane of the bottom of the feet, it is therefore possible to have four vertical rods, parallel to one another, attached to the same base. This is a great convenience in assembling complicated combinations of apparatus. Rods smaller than the holes can be clamped by means of bushings. When these bushings are of non-conductors the rods can be insulated, or the rods may be made of these materials.

The smaller base has all the features of the one just described, and weighs about three fourths of a kilogram. The holes in the feet are 10 mm. in diameter, and the central one 13 mm. These bases can be bored while clamped together in pairs. This insures exact correspondence of the holes when the bases are used together in combinations. They form excellent end supports for the horizontal rod of an optical bench, or similar apparatus. They may be used instead of flanges for table tops and wheels. They can be fastened easily to the wall or ceiling by screws passing through the holes in the feet, or be employed in the construction of a wall bracket of adjustable height. In order to make such a bracket two short rods in two of the supplementary holes are held in corresponding holes in a block of wood screwed to the wall. A long rod in the anterior leg terminates in a rectangular piece through which passes a horizontal rod abutting against the wall. A table top is attached to the central standard when an adjustment for height is desired, or it may be screwed or clamped to the anterior rod. FREDERICK W. ELLIS

MONSON, MASS.

FRIDAY, NOVEMBER 2, 1917

CONTENTS

THE STRUCTURE OF ATOMS, AND THE EVOLUTION OF THE ELEMENTS AS RELATED TO THE COMPOSITION OF THE NUCLEI

OF ATOMS1

THE general theory of the structure of the atom which seems to be most closely in harmony with the facts is that developed by Rutherford. His theory assumes that the atom consists of a central nucleus or sun, and that the satellites of the miniature solar system are the negative electrons. The central nucleus is supposed to contain almost all of the mass of the atom, and is charged with positive electricity. That this nucleus is very minute in comparison with the size of the atom is indicated by the work of Rutherford, of Geiger and Marsden, and of Darwin, who find that the deflection of alpha particles, which are shot from radioactive atoms at speeds which approach 20,000 miles per second and so pass directly through other atoms, is of such a character as to indicate that the positive charge of the atom is very highly concentrated. Thus Darwin's work indicates that the maximum diameter of the nucleus of a hydrogen atom (1.7 × 10-13 cm.) is only about one-one hundred thousandth of the diameter usually assumed for the atom. On this basis the atom would have a volume a million-billion times larger than that of its nucleus, and thus the nucleus of the atom is much smaller in com

1 Address presented at the Symposium on the . Structure of Matter at the New York meeting of the American Association for the Advancement of Science. A bibliography will be found in the following papers: Jour. American Chemical Society, 37, 1367-1421 (1915), 39, 856-879 (1917); Philosophical Magazine, 30, 723-734 (1915).

parison with the size of the atom than is the sun when compared with the dimensions of its planetary system.

How

The special modification of Rutherford's theory which has met with the most success is that due to Bohr. The very remarkable features of this theory have been made the subject of Professor Millikan's address, which has already been given, so they need not be mentioned here. ever, in spite of its success, Bohr's theory possesses in common with the other special views of atomic structure which have been developed, the limitation that its application has been restricted to one special class of phenomena, those of radiation, and that it is too simple to give a mechanism which I will act as any except the most simple of atoms. In the Bohr atom the negative electrons external to the nucleus are all supposed to lie in the same plane with the nucleus, while the structural relations of organic molecules seem to indicate that at least the outer electrons do not lie in a plane (except when only two in number) but that they have a three-dimensional arrangement.

It was found by Moseley that if the elements are arranged in order according to their X-ray spectra, they fall in the same order as they do in the periodic system. If arranged in this way, beginning with hydrogen as 1, and helium as 2, they are said to be arranged according to their atomic numbers. In our ordinary system of elements there are in all 91 elements from helium to uranium inclusive, and in addition to these there is hydrogen which makes 92 in all. Of these 86 or 87 have been discovered and 6 or 5 remain to be found. It is the purpose of this paper to present some relations which have been found by the writer and his students, which have a bearing on the structure of the atoms of these elements, upon the problem of their stability, and their formation by evolution.

1. ARE THE ELEMENTS INTRA-ATOMIC COMPOUNDS OF HYDROGEN?

One of the first suggestions in regard to the structure of the atom was made by Prout in 1815, or a little over a century ago. Prout found, on the basis of his own experiments and the more accurate work of Gay-Lussac, that if the atomic weight of hydrogen was put as 1.00, the atomic weights of the other elements became whole numbers as follows:

PROUT'S ATOMIC WEIGHTS (1815 A.D.) (WITH THE 1915 ATOMIC WEIGHTS ON HYDROGEN BASIS IN PARENTHESES)

If Prout's atomic weights had proved exactly correct, his claim that hydrogen is the protyle (porn ún) or fundamental element, might have seemed justified, but when it was found that these weights were very far from correct his hypothesis was largely discarded.

The prejudice which existed a few years ago against Prout's idea is well shown by a quotation from von Meyer's "History of Chemistry," printed in 1906.

During the period in which Davy and Gay-Lussac were carrying on their brilliant work, and before the star of Berzelius had attained to its full luster, a literary chemical event occurred which made a profound impression upon nearly all the chemists of that day, viz., the advancement of Prout's hypothesis. This was one of the factors which materially depreciated the atomic doctrine in the eyes of many eminent investigators. On ac

count of its influence upon the further development of the atomic theory this hypothesis must be discussed here, although it but seldom happens that an idea from which important theoretical conceptions sprang, originated in so faulty a manner as it did.

However, a careful study of the most accurately determined of the recent atomic weights reveals some very remarkable relationships. If first of all we make the assumption, as a subject for argument, that the heavier atoms are built up from hydrogen atoms, then it is found that the atoms are in nearly all cases lighter than they should be on the basis of such an hypothesis. Thus, if the following atoms of low atomic weight are considered, it is found that nearly all of them weigh 0.77 per cent. too little.

Therefore, if these atoms are built from hydrogen atoms, there must be during their formation a loss in weight, and presumably in mass, equal to 0.77 per cent. This will be called the "packing effect." When all of the 26 elements from helium of atomic number 2, to cobalt (No. 27) are considered, it is found that with the exception of the four elements, beryllium, magnesium, silicon, and chlorine, which have atomic weights higher than the corresponding nearest whole numbers, the average packing effect of the elements is again -0.77 per cent. This constancy of the packing effect suggests that the variation is due to some single cause, though the four exceptional cases cited above, show that there is undoubtedly some other compli

cating factor. The discovery by Thomson and Aston that the similar exceptional case of neon is due to the admixture of an isotope of higher atomic weight suggests that it may not be impossible to find a cause for the exceptional behavior in the four other

cases.

It has formerly seemed difficult to explain why the atomic weights referred to oxygen (16.00) as a basis are so much closer to whole numbers than those referred to hydrogen as 1.00, but the explanation is indeed very simple from the point of view presented here. The closeness of the atomic weights on the oxygen basis to whole numbers, is indeed extremely remarkable. Thus for the eight elements from helium to sodium the average deviation is only 0.02 unit, or less than the average probable error of the atomic weight determinations, and for all of the first 27 elements the average deviation from a whole number is, though more, increased only to 0.09 unit, when the sign of the deviation is considered. If it is not considered the deviation is reduced to 0.01 unit for 21 elements. The probability that such values as these could be obtained by accident is altogether unworthy of consideration. If an oxygen atom is a structure built up of 16 hydrogen atoms, then according to the ordinary theory that mass and weight are strictly additive, the weight of an atom of oxygen should be exactly 16 times the weight of a hydrogen atom. Now, according to the present system of atomic weights the weight of an atom of hydrogen is taken as 1.0078, so the oxygen atom should weigh 16.125. However, it is found to weigh 16.00. The difference between 16.125 and 16.000 is the value of the packing effect, and if this effect were exactly the same for all of the elements except hydrogen, then the choice of a whole number as the atomic weight of any one of them, would, of necessity, cause all of the other atomic weights

to be whole numbers. Though this is not quite true, it is seen that the packing effect for oxygen is 0.77 per cent., which is the average packing effect for the twenty-one elements considered (elements of low atomic number). Therefore these elements, which have packing effects equal to that of oxygen, will have whole numbers for their atomic weights. Since, too, the packing effect is very nearly constant, all of these 21 elements will have atomic weights close to whole numbers.

While according to our ordinary experience mass and weight seem to be additive, the question may be raised whether in the formation of atoms, which is a process which is, up to the present time, outside our experience, this is true. There are three remarkable facts to be explained: first, the atomic weights of the lighter elements on the hydrogen basis approximate whole numbers; second, the deviations. from whole numbers are negative, and third, these deviations are practically constant in magnitude.

It has been already stated that according to the work and calculations of Darwin, and of Geiger and Marsden, the nucleus of the atom is extremely minute in comparison with the size of the atom, so that in the nucleus the mass, if the determined dimensions of atoms and their nuclei are at all correct, is many thousand billion times more concentrated than in the atom. If the nucleus is complex, the electromagnetic fields of the charged particles would be extremely closely intermingled in the nucleus, and it would seem reasonable to assume that this would affect the mass, so that the mass of the whole nucleus would not be equal to the sum of the masses of its parts.

Let us take an extremely simple case for calculation, and find how closely packed the charged particles in a nucleus would have to be to cause the observed decrease in

weight (0.77 per cent.) which is found for most of the atoms. In making such a calculation, as a guide for our assumptions, we have the observed fact that radioactive atoms shoot out both positively charged alpha particles and negative electrons at such high speeds that it seems probable that they come from the nucleus of the atom. The observed relations between the products of the radioactive changes support this idea very strongly indeed. Thus there seem to emerge from the nuclei of complex atoms both positively and negatively charged particles, and the negatively charged particles are found to be negative electrons. This point should be emphasized, since many workers on atomic theory have endeavored to construct their imaginary nucleus of positively charged particles alone.

The simplest case for calculation2 would then be for a nucleus consisting of one positive and one negative particle. Let the distance between the particles be d, the charges respectively e, and e,, let the ve locity of the particles be along the straight line connecting them and equal to u. Then if c is the velocity of light, the particles have a longitudinal momentum which differs from the momentum calculated by ordinary mechanics for electrically neutral particles possessing mass by an amount equal to