266 
large to influence the result are of such rare occurrence that they 
may be neglected altogether. In so far as the fact that the field ot 
a doublet does actually extend to infinity introduces difficulties into 
the treatment, we shall, where necessary, conceive that the field is 
annihilated at distances greater than r==oo so that we may regard 
rt as the radius of the sphere of influence of the doublet. 
Ns ; d : 
The —- pairs we shall have to separate into various groups. We 
shall determine a definite pair of molecules in the following way: 
1 by the distance r between the centres, 
2 by the angles 6, and 6,, which the axes of the doublets make 
with the line joining their centres. In this we shall choose the direc- 
tion of a line joining the centres of two molecules as positive for 
the first molecule when it goes towards the other, and as positive 
direction of the axis of the doublet the direction towards the positive 
pole. The angle concerned will be taken as lying between O and z; 
3 by the angle g between the two planes each of which con- 
tains the axis of one of the doublets and the line joining their centres. 
Fig. 4. 
Values lying between O and 2x will be given to this angle. We 
can specify the angle g uniquely for any pair of doublets in the 
following way: Let AA’ and bb’ (fig. 4) represent the pair of 
doublets, A and B being the centres of the molecules, and A’ and 
B’ being in the positive direction along the axes of the doublets. 
Let us now take up our position either at A or B, say at A, and 
project AA’ and BL’ upon a plane passing through 6 and perpen- 
dicular to AB; the angle p is then that angle through which the 
projection of BB’ must turn in the positive direction as seen from 
A in order to coincide with the projection of AA’. 
The number of pairs of molecules which “are present” in a 
definite element of the space determined by the coordinates 7, 6,, 
0,, p, that is to say, the number of molecule pairs with a definite 
“space freedom (drd0,d0, dp)” we shall indicate by putting the 
