[ce is formed by the expulsion from the water of the circular, and the compression of the scalene and acute-angled atoms contained in it ; further by the accretion of such atoms from without, which being driven together cause the water to solidify after the expulsion of a certain number of round atomsj [H_The rainbow arises when the sun shines upon hum id air ; or again by a certain peculiar blending of light with air, which will cause either all the distinctive qualities of these colours or else some of them belonging to a single kind, and from the reflection of this light the air all around will be coloured as we see it to be, as the sun shines upon its parts. The circular shape which it assumes is due to the fact that the distance of every point is perceived by our sight to be equal ; or it may be because, the atoms in the air or in the clouds and deriving from the sun having been thus united, the aggregate of them presents a sort of roundness^/ " A halo round the moon arises because the air on all sides extends to the moon ; or because it equably raises upwards the currents from the moon so high as to impress a circle upon the cloudy mass and not to separate it altogether ; or because it raises the air which immediately surrounds the moon symmetrically from all sides up to a circumference round her and there forms a thick ring. And this happens at certain parts either because a current has forced its way in from without or because the heat has gained possession of certain passages in order to effect this. ! t Comets arise either because fire is nourished in 6$7