40 PRESIDENT'S ADDRESS—SECTION A. 
symbols—often traps for the unwary (7), or pushed back until 
they might be imagined to have a reasonable chance of escaping 
detection, is a singular feature in the history of natural philo- 
sophy. 
Generation of Motion in Mass.—Historically there are but 
three different ways, two of them being fundamentally different, 
in which motion has been imagined to be imparted from one 
body to another, by those who espouse the mechanical method 
of interpreting the universe. The first and most popular is by 
impact. If the impacting atoms (/) are conceived as absolutely 
hard—if that be intelligible—that is, as having, say, a co- 
efficient of restitution of unity | ?], no energy will be lost through 
impact, as experience would suggest, but by definition, such 
atoms are not subject to elastic deformation, and, therefore, can- 
not vibrate. Consequently, as is well known, they are incom- 
petent to discharge their explanatory functions, because certain 
phenomena involve the assumption that atoms do vibrate. On 
the other hand, if the atom be conceived as subject to elastic 
deformation, and to escape the difficulty of an infinite series is 
also supposed to be structureless, continuous, or non-molecular, 
then it may be said it is unlike any existing matter, or anything 
which has come within the range of sense perception. It is a 
supersensible material, and anything more than a purely 
empirical and formal theory of its deformation and general 
behaviour would be impossible. All explanations of the elastic 
properties of matter, which are not purely formal, that is, 
which take account merely of spatial distributions of energy 
without inquiring into the explanation thereof, presuppose mole- 
cular structure, with attractions and repulsions, and so on; 
molar examples of elastic deformation being regarded as involv- 
ing such conceptions in any complete explanation. This exem- 
plifies the method of eliminating a difficulty at one place to 
reintroduce it at another. That the linear scale of the element 
into which it has been transferred is relatively infinitesimal in 
no way alleviates the specific character of the difficulty. The 
fact obviously is that either the absolutely hard, or perfectly 
elastic atom, has a purely hypothetical existence ; the properties 
assigned to it, by mere transfer from the sphere of sense percep- 
tion, themselves require explanation as soon as they are regarded 
as derivative, and not ultimate in that sphere. 
The Aither as the Medium of Communication of Motion.— 
The remarkable practical utility of the conception that the 
ether, by the establishment of systems of stresses therein, may 
(j) In this connection Magnus, who played so prominent a part in the establishment of 
a school of scientists in Berlin, may be mentioned. One wonders how far his opposition 
to mathematics, beyond all question an indispensable and masterful instrument of 
physical research, arose from an appreciation of its occasional misuse. 
(k) Cases of molar impact need not be considered, as the phzenomenon is admittedly 
not then elementary in its nature. 
