1888 - 89 .] Sir W. Thomson on Constitution of Matter. 
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collision — that is to say, can combine. It will certainly be a very 
rare incident that they remain for any considerable time in collision ; 
but I cannot prove that the case may not occur in which none will 
go free, and the three will remain in combination. 
§ 34. If the initially-given velocities are very great, the general 
result, even of triple collisions, will be to leave the individual atoms 
free. The comparatively rare double atoms resulting from triple 
collisions, and the still rarer triplets, will be liable to be separated 
again into single atoms by all fresh collisions. This is the case of a 
perfect monatomic gas, at a temperature much higher than the 
Andrews’ critical point. 
§ 35. But if the originally-given velocity be exceedingly small, 
the result of exceedingly nearly every triple collision will be to form 
a combination of at least two of the three colliding atoms. Imme- 
diately after the collision by which it was formed, each doublet will 
generally have considerable relative motion of its two atoms ; that 
is to say, the two will describe orbits round their common centre of 
inertia : or, in the extreme case of no moment of momentum round 
this point, they will oscillate relatively to their centre of inertia to 
and fro in a straight line ; the centre of inertia itself generally 
having a considerable velocity. Still supposing the average velo- 
cities of the free atoms to be very small, and their number to be 
very great in comparison with that of the double-atoms, we now see 
that the general effect of the collisions between double and single 
atoms must be to diminish the energies of the relative and absolute 
motions of the constituents of the doublets, and so reduce the 
doublets more and more nearly to the condition of pairs of atoms in 
relative equilibrium (§ 16 above), a,t distance £ asunder, with 
centre of inertia of each pair moving very slowly through space. 
§ 36. But now consider the effect of a collision between two 
doublets each with little or no intestine commotion before the 
collision, and with its centre of inertia moving very slowly through 
space. The case in which the same description would be applicable 
to the four atoms after the collision, whether in the same pairs or 
in interchanged pairs, would be exceedingly rare. So also would 
be the case of the four atoms remaining combined. The result in 
exceedingly nearly every case would be a triplet with considerable 
intestine commotion, and its centre of inertia moving rapidly 
