184 
MR. A. A. GRIFFITH ON 
spontaneously by the amount of the surface energy of the cracks. This view must 
therefore be regarded as untenable. 
Again, the observed weakening might conceivably occur if at any instant the vibrations 
of a large number (at least 10 8 ) of near molecules synchronised and were in phase, 
provided the energy of these molecules was approximately that corresponding with 
the temperature of ebullition of the substance. Except in the case of a material very 
near its boiling point, the probability of such an occurrence must be so small as to be 
quite negligible. Hence this hypothesis also must be discarded. 
The foregoing discussion seems to suggest that the assumed type of molecule is too 
simple to permit of the construction of an adequate theory. An increase in generality 
may be obtained by supposing that the attraction between a pair of molecules depends 
not only on their distance apart, but also on their relative orientation. The properties 
of crystals seem definitely to require the molecules of anisotropic materials to be of 
this type, but those of isotropic substances have usually been assumed to be of the 
simpler kind. In view of the author, however, molecular attraction must be a function 
of orientation even in substances, such as glass, metals and water, which are usually 
referred to as “ isotropic.” 
Consider a solid made up of a number of such molecules, initially oriented at random. 
Doubtless the mechanical properties of the substance, wdiile it is in this amorphous 
condition, will differ little from those of a substance composed of molecules of the 
simpler type, having an attraction of appropriate strength. If this is so, the tensile 
strength of the material must be that corresponding with its average intrinsic pressure. 
In general, however, this initial condition cannot be one of minimum potential 
energy. 
It is clear that under suitable conditions the tendency to attain stable equilibrium 
can cause the molecules to rotate and set themselves in chains or sheets, with their 
maxima of attraction in line. The formation of sheets will commence at a great number 
of places throughout the solid, i.e., wherever the initial random arrangement is sufficiently 
favourable. Evidently it is possible for the number of such “ sufficiently favourable ” 
arrangements to be enormously less than the total number of molecules, so that the 
ultimate result will be the formation of a number of units or groups, each containing 
a large number of molecules oriented according to some definite law. The relative 
arrangement of the units will, of course, be haphazard. 
Now, in each unit there will, in general, be a direction which is, approximately at 
least, that of the minimum attractions of the majority of the molecules in the unit. 
Hence if rhe ratio of the maximum to the minimum attractions is sufficiently great, 
each unit can constitute a “ flaw,” and there aj)pears to be no reason why the units 
should not be as large as the flaws have been shown to be in the case of glass. Thus, 
in order to explain the spontaneous weakening of glass, it is only necessary to suppose 
that the thermal agitation at about 1400° C. is sufficient to bring about the initial 
random formation. 
