Bicrerton.—0n Partial Impact. 127 
side, and the heavier on the inside of bodies. Hence, the chief elements of 
the surface of bodies may reasonably be expected to be hydrogen, lithium, 
carbon, nitrogen, oxygen, magnesium, sodium, and sulphur. All these 
elements, except lithium (which may consequently be assumed to be 
universally rare), are the common elements of the surface of bodies; and 
hydrogen, the highest of all known bodies, is the most common of all. Is 
not the element of 1474 line, which is found outside of hydrogen on the 
sun, an element of still less atomic weight than hydrogen? If this 
hypothesis be true, then it is reasonable to assume that diffused hydrogen 
must fill space. This would account for the retardation of comets and 
planets without the assumption of an ether resistance. It thus appears 
that the molecular motion of gases may become one of mere translation. 
There is accordingly a continuity of heat and mechanical motion. It is 
reasonable to suppose, that at a certain height above the sun the general 
motion of the particles of hydrogen may become more or less parallel ; there 
would be no collisions of molecules, and consequently no luminosity would 
be then produced, and an apparent dissipation of the protuberances would 
occur. I have now shown the most striking points in the contrast of the 
energy of different cases of collision. I have also shown a possible reason 
why the small atomic weight elements are common on the surface of 
bodies; why we should expect to find hydrogen on the surface of all 
bodies, such as the sun and stars; lastly, that hydrogen, and probably the 
unknown element of the sun, may be the resisting substance which retards 
the motion of bodies in space. 
On the Rotation of Systems. 
It does not seem reasonable to expect rapid rotation in the case of entire 
coalescence of two bodies, as only the resultant of the two original rotations 
will tend to develope this motion. But, in the case of partial collision, we 
must have a rapid rotation of the mass, as each of the two bodies from 
which it was formed occupy chiefly one side of the new body, and as the 
velocity of each of the two bodies was originally opposite to that of the 
other, rotation is a necessary consequence. 
There are two chief reasons for the inequality of the balance of mo- 
mentum at the two sides of the coalesced mass: 1st. The piece cut off will 
be much thicker towards the middle of the original mass than at the out- 
side. 2nd. The density of the inside is much greater than that of the 
outside, in consequence of the greater pressure, and also from the fact that 
it is probable the heavier elements are towards the centre of the mass. It 
may easily be seen that the resultant momentum on the two opposite sides 
are in opposite directions, consequently tending to rotation, . 
