At present, unanimity of opinion, even on questions of the most primary kind, is far to seek. Philosophers are not even agreed as to the constitution of the nebulæ. It is questioned whether even those least resolvable and most diffused forms which give bright line spectra really consist of masses of incandescent gas. Many observers, among them Sir Norman Lockyer, now maintain that they are formed of swarms of meteorites, which, moving with prodigious velocity, meet in frequent collision, and by their impact evolve sufficient heat to become self-luminous. Others, again, like the distinguished investigator Arrhenius, while admitting the gaseous nature of these nebulæ, deny that they are incandescent, and assert that their temperature is not much above that of surrounding space. Their exterior parts consist of the lighter gases in a highly rarefied state, and minute particles of negative electricity, which are always careering through space, on penetrating these gases produce a luminous discharge. A nebula composed of swarms of meteorites would, as Sir George Darwin has shown, behave very much in the same way as one composed of gas, and if in rotation would rotate as a solid mass. The meteorites would stand in the same relation to the nebula as molecules to a gas, and thus the question of the constitution of the nebula, although of great interest in itself, becomes of subsidiary importance in tracing its subsequent history. [Sidenote: Shaping of the Planets] One of the latest attempts to frame a nebular hypothesis is that of Professor J. H. Jeans. His reasoning is of a highly mathematical character, and his conclusions are expressed in the most general terms. Starting with a spherical nebula of gas or meteorites endowed with a small amount of rotation, he shows that as it cools or loses energy the temperature of the interior will not fall continuously in precise correspondence with the cooling of the outer parts, and this “lag” of the interior temperature will bring about a tendency to instability. The contraction of the nebula due to cooling will increase the velocity of rotation, and this again will tend to instability. As a result of the instability so produced the nebula will change its form, and become more or less pear-shaped. The narrow end of the pear will then separate from the body and assume an independent existence as a primitive planet. This process will recur again and again till the nebula is resolved into a sun with its attendant planets. The planets, existing at first as gaseous masses or quasi-gaseous masses, will be liable to the same kind of transformation, and may thus bud off moons or satellites.