was exclusively the seat of electromagnetic fields. According to
Lorentz the elementary particles of matter alone are capable of
carrying out movements; their electromagnetic activity is entirely
confined to the carrying of electric charges. Thus Lorentz succeeded
in reducing all electromagnetic happenings to Maxwell's equations
for free space.

As to the mechanical nature of the Lorentzian ether, it may be said
of it, in a somewhat playful spirit, that immobility is the only
mechanical property of which it has not been deprived by H. A.
Lorentz. It may be added that the whole change in the conception
of the ether which the special theory of relativity brought about,
consisted in taking away from the ether its last mechanical quality,
namely, its immobility. How this is to be understood will forthwith
be expounded.

The space-time theory and the kinematics of the special theory
of relativity were modelled on the Maxwell-Lorentz theory of the
electromagnetic field. This theory therefore satisfies the conditions
of the special theory of relativity, but when viewed from the latter
it acquires a novel aspect. For if K be a system of co-ordinates
relatively to which the Lorentzian ether is at rest, the
Maxwell-Lorentz equations are valid primarily with reference to K.
But by the special theory of relativity the same equations without
any change of meaning also hold in relation to any new system of
co-ordinates K' which is moving in uniform translation relatively
to K. Now comes the anxious question:--Why must I in the theory
distinguish the K system above all K' systems, which are physically
equivalent to it in all respects, by assuming that the ether
is at rest relatively to the K system? For the theoretician such