52 Proceedings of Royal Society of Edinburgh. [sess. 
molecules. Before the union we may conceive them flying about 
in space in much the same way as afterwards, coming into collision 
and flying apart, in the majority of cases, with the violence of the 
rebound ; hut here and there one would catch another up from 
behind, and the union would be so gentle, and the force binding 
them together after union would be so great, that they could not 
separate. In course of time they would all pair. But why pair 'l it 
will be asked. Why not join in threes and fours, etc. ? For a very 
good and substantial reason, I reply. When two atoms unite they 
cling together more or less forcibly or closely, dependent on their 
sizes and squeezability; the more closely they cling, the more are they 
flattened together at the point of contact, thus crowding the par- 
ticles of which they are formed closer together, and so rendering the 
atoms less squeezable. In this state it is far easier for an unpaired 
atom to unite with another unpaired one, because the paired ones 
are made by their union so rigid that it is difficult for another to 
gain and retain a hold upon them. The area of contact between 
such would be so comparatively circumscribed, owing to the 
rigidity of the paired atoms, that on the first opportunity the odd 
one would unite with another unpaired one, and so on until they 
were all paired. 
It is evident that the larger-sized atoms in proportion to mass — 
the more open-grained, in fact — would cling closer together in 
forming molecules; their particles having more space between 
them, could be made to crowd closer together, and so admit of a 
greater surface-contact. But there would be a limit to this. — 
See Valency. They might be so large-sized in proportion to 
atomic weight or mass, and their union would be so close, as to 
admit of only, owing to their small weight, an almost insensible 
quantity of vibration. The total heat of a gas is made up of two 
quantities, viz., the internal vibration of the molecules and their 
translatory motions within the space occupied by them. The heat 
of some gases may be almost entirely made up of translatory 
motion, and comparatively little of it of vibratory. Tyndall 
found* that the absorption of radiant lieat by the simple gases 
hydrogen, oxygen, and nitrogen was almost insensible, and also 
that their radiation was on the same level. How, if we assume 
that the atoms of these gases are so very large in proportion to 
* Heat, a Mode, of Motion, p. 346, 6th ed., 1880. 
