96 A TREATISE ON METAMORPHISM. 



on glass. If a piece of glass, free from stress, be placed under the micro- 

 scope with crossed nicols, the light is cut off because the glass is isotropic. 

 If, however, the glass be slightly flexed, well within the elastic limit, it 

 immediately becomes anisotropic, and brilliant colors flash out. So far as 

 light is concerned — and this is one of the best agents for giving an insight 

 into the molecular constitution of bodies — the strained glass behaves wholly 

 different from unstrained glass. Evidently when glass is alternately strained 

 and freed from strain it undergoes a profound change in molecular consti- 

 tution. The greatness of the molecular change in material when strained 

 within the elastic limit is dwelt upon to show that such changes might 

 greatly affect chemical action; and it will be seen below that the facts 

 correspond to this expectation. 



Barus has shown" in the case of metals strained to the point of rupture 

 that a considerable per cent of the energy expended in straining them is 

 potentialized; in "glass-hard" steel 50 per cent, in brass 40 per cent, in 

 copper 25 per cent. A larger percentage of the energy was potentialized 

 in the earlier stages of strain than in the later stages. By stating that 

 energy is potentialized is meant that the mechauical equivalent in heat of 

 the work done on the metals was only partially developed ; the remainder 

 of the energy is stored up in the strained metals. Now, considering a brittle 

 substance which is analogous in physical characters to rocks, Prince Rupert 

 drops, the explosion of a drop when a point is broken shows that a large 

 amount of energy is potentialized, or that the glass is in a high state of 

 strain. The experiments of Barus and the condition of the Rupert drop 

 show that in strained materials energy is probably potentialized. If this 

 be true, must it not be the case that the atoms and molecules of a strained 

 body are in a more than ordinarily favorable condition for chemical action? 



Bodies in which energy is potentialized are believed to be in an 

 exceptionally favorable condition for chemical action. For instance, if a 

 strained metal, in which on that account more than the usual amount of 

 energy is stored, be dissolved in an acid, less than the usual amount of 

 chemical energy is expended, for the resultant salts in the solution have the 

 same energy of combination in each case. But in the strained metal work 

 has been done, the equivalent of which has not. escaped as heat during 

 strain, and is therefore stored energy. Therefore this energy is available 



"Barus, C, The mechanism of solid viscosity: Bull. U. S. Geol. Survey No. 94, 1892, pp. 107-108. 



