Berenice's Hair, seen edgewise, appears as a line with a thickened centre. Laplace's theory explained why, with a few insignificant and intelligible exceptions, all the bodies of the solar system travel round the sun in the same direction and nearly in the same plane, for it represents them all as having been part of one lens-shaped mass.
The spectroscope added striking support to the nebular hypothesis by Huggins' discovery in 1864 that the nebula in Draco consists of incandescent gas; while later proof of the solidity of the material in spiral nebulae is consistent with their representing a later stage than that of the nebula in Draco. Such nebulae would include white-hot particles, though the average of the mass would be cold. The spectroscope also revealed the fact that the different h;avenly bodies consist of similar, materials. Most of the terrestrial elements are known in the sun, in which hydrogen, calcium, iron, carbon, and sodium are especially conspicuous. It is true that the only material yet recognized by the spectroscope in some nebulae is the light gas, nebulium, and that the solar corona consists mainly of coronium; both these elements are unknown on earth. Despite such exceptions the spectroscope has demonstrated the uniformity of material throughout the solar system and the general unity of matter.
Laplace's theory was therefore regarded as firmly established by astronomical observation finding so many agreements with its re- quirements. Weighty mathematical and physical considerations, however, tell against it. A cloud of white-hot gas as tenuous as a nebula and surrounded by the intense cold of outer space should cool almost instantaneously. The luminosity of the nebulae is one of their still mysterious properties. " We have no knowledge," says Hale (Stellar Evolution, 1908, p. 206), "why they glow with a steady and unchanging light, since there is no direct evidence that this light is produced either by heat or by electrical excitation."
The objections to the nebular theory which have carried the greatest weight are those founded on the distribution of energy in the solar system. Jupiter possesses only i/iooo of the total mass of that system, yet possesses 95 % of the total energy of rotation. The mathematical objections are regarded by Chamberlin, Moulton and See as absolutely fatal to the theory, while Jeans remarks (Prob- lems of Cosmogony, 1919, p. 274), " it seems probable, although by no means certain, that we must abandon the Nebular Hypothesis of Laplace." According to Hale (op. cit. 1908, p. 186) it must be re- constructed or abandoned.
The chief rival to the nebular theory is the meteoritic theory, which regards the various stellar systems as formed, not by the cool- ing of clouds of incandescent gas, but by the aggregation of innumer- able small meteorites. The meteorites coming together are heated by collision and pressure until they are fused into a solid mass. The meteoritic theory has three chief forms. The first, due to Sir Nor- man Lockyer, trusted to the infinite numbers of meteorites in space to provide ample material for the star systems. The numbers are indeed infinite. Any observer may see about seven per hour on any moonless, cloudless night, and from this number it is calculated that from 10 to 15 million enter the earth's atmosphere every day. It has been calculated that they occur through space on an average of 200 m. apart. Most of those which fall on the earth are minute, and they are pulverized by friction with the atmosphere into dust, the quantity of which is relatively so small that it would take aeons for the earth to add one cubit to its diameter by this process. The earth may, however, have formerly received more of this material, as it is travelling along a path which has now been swept clear of meteorites; it is chiefly those that are travelling on irregular paths through space which fall upon the earth, though Sir Norman Lock- yer recognized that many meteorites travel round the sun on regular orbits like minute planets. The objection that the number of meteorites is too small and gravity too weak to collect them into swarms led to a second form of the hypothesis the planetesimal theory of Chamberlin \Geology, vol. ii., p. 4, 1906). He assigned the chief constituents of the planets to meteorites which are moving in orbits around a common centre. He described them as " infinitesimal planetoids or planetesimals." The theory in this form had the attrac- tion that it explained the segregation of the meteorites, but in his later statement of it (Origin of the Earth, 1916) the planetesimals are relatively unimportant and the planets are attributed to clots formed in rays shot out from a star which has been disrupted by the close approach of another star. The constituents of these clots revolve around a secondary centre and are minute satellites, not planetesi- mals. If a star be thus disrupted two antipodal rays would be pro- jected from it, and during its rotation the ends of the rays would lag behind as in spiral nebulae; the segregation of the material in the rays into clots would produce the planets.
A third form of the meteoritic theory, that of T. J. J. See, represents space as originally crowded with meteoritic dust which has gradually collected around larger bodies and thus slowly built up star systems.
All varieties of the meteorite theory agree in assigning the origin of the earth to an aggregation of cold meteorites which have been welded into a solid body by collision and pressure. This theory seems to offer the most probable explanation of the origin of the earth, but the form that is most likely to gain acceptance is uncertain. As Mr. Jeans remarks (pp. cit. p. 288), " the time for arriving at conclusions in Cosmogony is not yet come." The evidence available, however, justifies the conclusions that the world is an iron projectile, hard- ened, like those of modern artillery, by nickel, and whirling through
space; that it was probably due to a swarm of meteorites, which fused into a compact mass; and that during its consolidation the metals settled downward into the core, the barysphere, while the lighter stony materials floated like a slag to the surface and there solidified the lithosphere or crust of the earth.
The Origin of Life on the Earth. The physical conditions on the early earth also bear on the problems connected with the origin of life on it, which has been explained as introduced from some other world. The conditions on the earth were probably as favourable to the independent generation of life as any we can assume elsewhere. The formation of living from dead matter by processes still in operation has been claimed as probable by Sir E. A. Schafet in his presidential address to the British As- sociation (1912); but it more probably happened in the early days of the earth under the special atmospheric and climatic conditions which prevailed in primeval times.
Living matter consists mainly of compounds of carbon, many of which have been made artificially. The carbohydrates can be made by inorganic processes and also altered into the more complex amino acids, which are the physical basis of living tissues. A carbonaceous jelly which has many of the properties of living matter can be made artificially, and would probably have been formed naturally under the conditions prevailing when the earth was young. The vitalization of this inorganic material would have resulted from the formation of a catalytic agent concentrated in a small spot or nucleus, which would control the subdivision of the growing mass of jelly when its size became too large to be stable. Chemical research on the carbon com- pounds has shown that there is no insuperable difficulty in the production of organic materials from non-living matter, without the intervention of a living organism. The process by which the non-nuclear carbonaceous jelly was developed into the primitive organism Protobion, as it has been called (Gregory, Making of the Earth, 1912, p. 231), by the development of a nucleus which acts catalytically probably through some phosphoric constituent, is the gap still left in explaining the origin of life. The environment on the muddy shores of the primeval sea, when the atmosphere was richer, warmer and moister than that of the present day, and changes of temperature were slight, would have been especially suitable for this momentous development.
Geological History in Relation to Cosmic Theories. Life once established would probably have developed steadily since the earth's surface comparatively soon reached an average tem- perature and climatic conditions which have persisted through- out later geological times, though there may have been rhythmic minor oscillations. The geological history of the earth is better concordant with the meteoritic than with the nebular hypothesis. On the latter would be expected a series of progressive changes throughout geological time, the climate becoming colder as the sun and interior of the earth cooled, volcanic activity diminishing as the crust thickened and the surface of the fluid rock material lay deeper, earth movements losing vigour though generally following the same lines, and the oblateness of the earth increas- ing as the equatorial bulge enlarged under the unceasing in- fluence of rotation. It has also been held that the age of the earth must be comparatively short owing to the limited heat supply in the solar system.
Calculations as to the age of the earth based upon the rate of the loss of heat from the sun led to the belief that its duration must be relatively short, and the estimates have varied from possibly as little as 13 million years to perhaps as long as ica million years. The recognition of other sources of heat, such as atomic disintegration, has, however, released geology from the short mathematical limits on the age of the earth.
In a paper by Prof. H. N. Russell, read before the Royal Society, March 10 1921, the estimated age of the earth was given between 2,000 million and 8,000 million years. Geologists and cosmogonists now assume practically as great an age for the earth and the solar system as they find convenient, and i ,000 million years is a moderate minimum.
The progressive changes that would appear to be the natural consequence of the nebular theory are not indicated by geological evidence. Instead of a steady advance throughout the ages there
