PRESIDENTS ADDRESS—SECTION A. 29 
22. The “ Strain Figure in Solid Ather” Theory.—As late as 
1891 another theory was propounded which also postulates an 
indefinitely extended medium, but of a solid or quasi-solid 
character. That is to say, no part of the medium is regarded 
as subject to translational motion, even of the atomic order of 
minuteness. This theory was set forth by Dr. Burton (0). It may 
be thus stated:—-The ether is an indefinitely extended quasi- 
solid medium, sensibly perfect in its elastic properties [7.¢., free 
from viscous yielding and internal friction], in which ever-chang- 
ing distributions of stress and strain account for all phenomena. 
Assuming that the zther, hypothetically conceived to be origi- 
nally homogeneous and isotropic, might by some compelling 
agency be so strained that the restoring stresses, instead of in- 
creasing with the strains should fall off, a state would be reached 
when the withdrawal of the compelling agency would leave the 
medium in a new condition of stable equilibrium, involving 
stress and strain at every point. An atom is conceived to be, 
not an individual portion of the zther, but one or more of these 
stressed modifications, called by Burton strain-figures, and the 
motion of an atom is not that of a definite portion of the ether, 
but the movement only of the strain-figure or system of strain- 
figures (c). An illustration, given in Professor Fitzgerald’s lec- 
tures, is the way in which a drop of water travels through a 
block of ice; that is to say, the zther is regarded as remaining 
eternally at rest, it is only the modification of structure or 
energy—the strain-figure—which moves about in it. Similarly 
the motion of a material body is simply the motion of a con- 
geries of strain-figures. 
23. Properties of Strain-figure Atom.—Since a strain-figure 
is originally in equilibrium, the transfer to some other portion 
of the medium, or its change of orientation involves no question 
of statical resistance or of the medium giving way. Burton 
shows that the equations of motion of such an atom are of the 
same form as those of a solid immersed in a perfect fluid ; that a 
strain-figure, symmetrical in respect to a given point, is dynami- 
cally equivalent to a mass particle thereat [l.c., p. 283; a result 
dynamically in conformity with the very different theory of 
Boscovich]|; that, provided the motions of atoms, molecules, or 
material bodies be slow compared with a certain critical velocity, 
say, that of gravitative action, they will encounter no 
[sensible?] resistance in travelling through the ether, and will 
obey laws of motion that include Newton’s laws as a particular 
case. Gravitative and inter-atomic forces may be supposed to 
(¥) On a theory concerning the constitution of matter. OC. V. Burton, Journ. Phys. 
Soe. Lond., XT. iii., pp. 275-290. Nov., 791. 
(c) It is recognised that atoms generally are formed by the aggregation of a large 
number of strain figures, since the spectra of the vapours of the elements imply a large 
number of degrees of freedom. 
