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APPENDIX I | 44 
4. SHOCK FRONTS 
that there is a limit to the steepness of the pressure gradient that can be propa- 
gated through a medium (11), (12). Waves of intense steepness can occur, however, in 
which conditions on the two sides of the steep gradient are related by the shock-front 
Equations, [11] to [14]. Such waves might be called physical shock fronts, in con- 
trast to the mathematical shock fronts just discussed. The thickness of a physical 
shock front should be of microscopic magnitude (for the method of estimating, see 
Reference (12)). 
The theoretical determination of the distribution of pressure within an in- 
tense physical shock front presents a difficult problem because the ordinary theory of 
viscosity and of the conduction of heat mey be expected to fail when the thickness of 
the front ceases to be large as compared with the distances between molecules. 
Furthermore, various forms of thermal hysteresis are likely to occur. 
A certain amount of experimental evidence exists in support of the theory 
of shock fronts, especially as to the occurrence of speeds of propagation much exceed- 
ing that of ordinary sound waves. 
Detonation waves in explosives are believed to be shock waves in which a 
chemical transformation occurs almost instantaneously as the wave passes. Because of 
the chemical change, an enormous rise of temperature occurs, and the pressure in- 
creases, as the wave passes, much more than it would owing to the increase in density 
alone. 
