5 
A rigid body compressed without fracture^ into the figure 
of W — was this possible? There is, as I afterwards dis- 
covered, a source of error in the word " rigid — a latent 
hypothesis which turns out to be erroneous. " Rigid is 
purely a relative term. Stone is rigid in comparison with 
clay, but plastic in comparison with cast steel. Absolute 
rigidity is an unknown property of matter. Let us select a 
few examples of what are commonly regarded as rigid bodies. 
Rock crystal, glass, calc spar, steel and limestone, are surely 
fair specimens. Yet Tyndall gives instances of quartz 
crystals altered in shape by pressure, some of them having 
yielded along transverse planes as if one half had slidden 
over the other, but subsequently strongly cemented together 
by mere apposition and pressure. He regards the action of 
strongly compressed glass upon polarized light as proof of an 
alteration in its molecular arrangement. Mr. Sorby has cited 
examples of distorted crystals of calc spar in cleaved lime- 
stones. M. Tresca, in his paper on the "Flow of Solids,'' 
read before the Institution of Mechanical Engineers at Paris 
in 1867, gives the results of experiments made upon lead, 
iron, and even steel, and shows that these metals behave like 
liquids when subjected to adequate pressures. As to lime- 
stones and other rocks, I can say from my own experiments 
that they are both elastic and plastic, yielding more or less to 
forces of short duration, but recovering their original figure ; 
while when subjected to long- continued pressures or strains 
of low intensity, they are capable of setting permanently 
in a new shape. 
It is curious to observe how speculation has been misled 
by the notion of absolutely rigid bodies, by the assumption 
that hard rock can exhibit neither an appreciable elasticity 
nor any ductile properties. Sir James Hall, of Dun glass, 
whom Professor Geikie has lately styled "the founder of 
Experimental Gfeology, since it w^as he who first brought 
