According to the view here adopted, this moment is caused by 
rotating or eireulating motions of the charges within the particle, 
similar to Amprrn’s molecular electric currents. Tf, for the product 
ed of a charge e and its velocity, we introduce the name of “quantity 
of motion of the charge”, the integral in (8) may be said to repre- 
sent the moment about the origin O of the quantities of motion of 
all the charges present. 
A very simple example is furnished by a spherical shell, rotating 
round a diameter, and enclosing av immovable, concentric sphere, 
the shell and the sphere having equal and opposite charges, uniformly 
distributed. 
Whatever be the motion of the charges whieh call forth the mo- 
ment m, we may properly apply to them the denomination of mag- 
netization-electrons. 
§ 5. In the determination of the mean values of the quantities 
in (1), (ID) and (1), the following considerations and theorems will 
be found of use. 
a. Consider a space containing an immense number of points Q, 
whose mutual distances are of the same order of magnitude as those 
between the particles of a ponderable body. Let NVN be the number 
of these points per unity of volume. If the density of the distribution 
gradually changes from point to point — in a similar way as may 
be the case with the observed density of a body — the value of 
N belonging to a point Pis understood to be derived from the 
number of points Q lying within a physically infinitely small space 
of which P is the centre. 
Draw equal and parallel vectors Q/? =r from all the points Q, 
and consider a physically infinitely small plane do whose normal, 
drawn towards one of its sides, is 7. The question is to find the 
number of the vectors QF that are intersected by the plane, a 
number which we shall call positive if the ends of the vectors, and 
negative if their starting points lie on the side of ds indicated by 7. 
If N has the same value throughout the whole space, and if the 
points Q are wregularly distributed, like the molecules of a liquid or a 
gas, the number in question will be the same for all equal and 
parallel planes, whence it is easily found to be 
LN BiB od a OO a ed es (10) 
The problem is somewhat less simple if the points Q have a 
regular geometric arrangement, such as those one considers in the 
theory of the structure of crystals. If, in this case, the length of 
the vectors QQ is smaller than the mutual distance d of neigh- 
