in the central portion of the stellar system, in which the sun seems to be placed at present, the general attractions are probably nearly balanced. Two stars, therefore, whose speeds are sub-equal and whose paths gently converge, may be controlled almost freely by their mutual attractions after they come within the spheres of each other's dominant influence. Such stars under mutual control would describe paths relative to each other similar to those assumed in the discussion. Their closeness of approach at periastron would be determined by the relative differences (not the total amounts) of their speeds and momenta. The principle of sub-parallel movements applies here and gives results quite at variance with those that obtain in cases of opposed movements, where the relative sums of the velocities and momenta are to be considered. The movements of the longorbit comets seem to be concrete expressions of this principle, as their perihelia are largely clustered on the front side of the Sun, i. e., the side toward which it is moving, and they make close approaches to it. Such star clusters as the Pleiades, the members of which seem to have proper movements nearly the same in amount and direction, are doubtless also expressions of the principle of sub-parallelism, and in their remarkable nebulosity they may at the same time illustrate the doctrine of disturbed secondaries leading on to dispersive action, a part of the product of which remains associated with the stars themselves, while a part is more or less widely scattered, as the terms of the doctrine require.

If our stellar system has a definite boundary and is a flattened spheroidal cluster or a discoid, and if the ideal paths of the stars are elongate orbits stretching from border to border across the heart of the cluster (except as diverted by close approaches), then the orbital speeds and momenta should be lowest on the outer surface, and the paths should there be most frequently sub-parallel, and hence the conditions for the close approach of two suns through their reciprocal attraction be there most favorable. Now, visible nebulæ are most frequent in the regions polar to the Milky Way, and they may be regarded as lying on

the flat sides or outer border of the stellar discoid where these conditions of low orbital velocity and momenta and prevalent sub-parallelism are dominant, and thus the distribution of nebulæ and the doctrine of close approach seem to be, so far at least, brought into harmony.

It may be needless to remark that the general conception lying back of the doctrine of dispersion by close approach has a complementary regenerative or reconstructive phase, which, taken with the dispersive phase, makes up a cyclic process. With the disruptive action there is correlated a reciprocal concentrative action, which is supposed to reproduce organized systems out of the wreckage of disrupted systems. The notion is further entertained that the two processes may be mutually self-adjustable, within the limits of general conditions, and thus may give a large degree of perpetuity to the existing phase of the stellar system.

UNIVERSITY OF CHICAGO,
June 1901.

T. C. CHAMBERLIN.

STUDIES FOR STUDENTS

THE CONSTITUENTS OF METEORITES.

ELEMENTS

THE following elements have by good authorities been reported as detected in meteorites by means of chemical or spectroscopic examination:

Many of these, however, occur only as traces, while others may possibly have been introduced by terrestrial agencies. The following list will be therefore more satisfactory as giving the primary and fundamental elements known to enter into the composition of meteorites:

It will be of interest to compare the more important of these in

the order of their relative abundance, with the eight most important elements of the earth's crust placed in similar order. The list of the latter is taken from Roscoe and Schorlemmer.'

It should be remembered in drawing conclusions from the above list that the elements of cosmic matter in its entirety are here compared with the elements of only the crust of the earth; further, that the meteoritic matter now known probably does not show a true proportion of stony matter. As I have shown elsewhere, the iron meteorites are much more likely to be known and preserved than the stony. It is probable, therefore, that if the average composition of meteoritic matter were known, iron would not occupy so high a place as it does in the above table. The relative excess of magnesium and nickel, and scarcity of aluminum and calcium in meteoritic, as compared with terrestrial, matter may be due to the same cause.

COMPOUNDS

The elements of meteorites chiefly occur combined. The exceptions are iron, nickel, cobalt, and copper, all of which occur largely in the form of alloys, carbon, and the gases, hydrogen, and nitrogen, probably held as elements in the pores of meteorites.

The compounds of meteorites according to the mineralogical names by which they are generally known, and roughly in the order of their relative abundance, are as follows, the minerals not occurring upon the earth being printed in italics :

I

Treatise on Chemistry, Vol. I.

2

JOUR. GEOL., Vol. V, p. 126.

A brief account will be given of each of these.

Nickel-iron. This is the most widely distributed constituent of meteorities and in quantity it exceeds all the others combined. It makes up practically the entire mass of all the iron meteorites, the larger part of the mass of the iron-stone meteorites and is found in nearly all, though not all, the stone meteorites. It is an alloy of iron and nickel in which the percentage of nickel varies from about 6 per cent. to about 20 per cent. Some iron masses claimed to be meteorites contain a higher percentage and some authorities regard the nickel-iron of most stone meteorites as generally containing from 20 to 40 per cent. of nickel, but this is somewhat uncertain. From 0.5 to 2 per cent. of cobalt always accompanies the nickel, as well as .006 to .02 per cent. of copper. Traces of manganese and tin are also often found. The terrestrial nickel-iron of the Greenland basalts differs from that of meteorites in having a lower percentage of nickel (0.25 to 4 per cent.) and in containing a considerable amount (3 per cent.) of carbon. The terrestrial nickel-irons known as awaruite and josephinite contain higher percentages of nickel than the meteoritic, the percentages being 67.7 per cent. and 30.5 per cent. respectively. In color, meteoritic nickel-iron varies from iron or steel-gray to silver-white, according to the percentage of