TRANSACTIONS OF SECTION A. 741 
for the known facts of retentiveness and coercive force and the characteristics of 
cyclic magnetic processes. 
. 4, That magnetic. hysteresis and the dissipation of energy which hysteresis in- 
volves are due to molecular instability resulting from intermolecular magnetic 
actions, and are not due to anything in the nature of frictional resistance to the 
rotation of the molecular magnets. 
5. That this theory is wide enough to admit explanation of the differences in 
magnetic quality which are shown by different substances or by the same substance 
in different states. . 
6. That it accounts in a general way for the known effects of vibration, of 
temperature, and of stress upon magnetic quality. 
7. That in particular it accounts for the known fact that there is hysteresis in 
the relation of magnetism to stress. 
8. That it further explains why there is, in magnetic metals, hysteresis in 
physical quality generally with respect to stress, apart from the existence of 
magnetisation. 
9. That, in consequence, any (not very small) cycle of stress occurring in a 
magnetic metal involves dissipation of energy. 
6. Some Experiments to determine Wave Velocity in certain Dielectrics. 
nd By Frep. T. Trovuron, 
The general method employed was described in ‘ Nature,’ January 1890. In 
Hertz’s well-known experiment of ‘ Loops and Nodes’ a sheet of a dielectric is 
inserted between the reflector and the resonator. The effect of this is to shift 
the system of loops and nodes towards the reflector. From the amount of this 
shift the index of refraction can be found. If the reflection from the surfaces 
be neglected p=“2—* 
where x, is the distance to the node in air or one quarter 
of the wave-length, and w the distance on inserting the sheet, 7 being the thickness 
of the sheet. A more complete formula, where the multiple reflections are con- 
sidered, as in Newton’s rings, is 
: l 
Q sin ct 
RCN aa nslaite 58" Yio dbo 
Fie 2 
ws (uw? —1) + (uw? +1) cos mpl 
Xo 
Experiments were made with pitch, solid paraffin, and sulphur. The index, as 
determined from the more complete formula for pitch, was found to be about 1:7 
(Hertz with his great pitch prism found about the same), but for sulphur and 
paraffin it came out quite too large—between 3 and 4. An explanation of this 
was put forward in the paper. The size of the reflector employed was so small 
that it introduced diffraction phenomena; that is to say, the nodes were situated 
further out than if an infinite plane were used as reflector (see ‘ Nature,’ August 
1889). This is caused by the increased velocity of the disturbance near a small- 
sized reflector. The explanation put forward maintains that this abnormal value 
of the velocity is effected in a greater ratio by the insertion of the dielectric sheet 
than the normal velocity would be. Taking the value of this velocity at distance 
Qo 2qe2 
ras V=v es , where v is the normal value, it was shown that the index or 
ratio of the velocities at distance » was greater than p, the normal index. This 
would explain why pitch came out satisfactorily, while paraffin and sulphur did 
not. For the pitch sheet was over 6 cm. thick, while the other two were only 
about 3 cm. 
Part of the paper contained an account of experiments on the absorptive 
powers of certain substances made by obtaining reflection from thin sheets. Thus 
glass and limestone 2 cm. thick afforded reflection. This is no doubt due to the 
absorption of the beam reflected from the back being so weakened by absorption 
