Physical and Chemical Behavior of Solids. 235 



Plastic sulphur. — Freshly prepared plastic sulphur supports 

 a pressure of 3000 atm. without immediate modification, but is 

 transformed in a few instants by a pressure of 6000 atm. 

 This change begins before the pressure has reached 5000 atm. 

 The surface of the block is then covered with a crust of 

 orthorhombic sulphur, while the interior remains plastic." * 



This evidence, while by no means conclusive, indicates that 

 a certain amount of transformation had taken place. Indeed 

 Spring, in a later article, admits this : " Yet this last trans- 

 formation (that of plastic into orthorhombic sulphur) requires 

 a long time. When the sulphur is taken out after being com- 

 pressed for some hours, it is still soft in the central part of the 

 cylinder. After undergoing compression for some days, the 

 transformation is complete; although a sample of plastic 

 sulphur, kept as a check under the same conditions of tempera- 

 ture, was not notably modified. "f 



The geueral conclusion to be drawn from this work is that 

 compression alone will not in general produce crystallization or 

 transform one modification of a substance into another ; it will 

 cause such changes in specific instances, where the conditions 

 are such that the velocity of transformation is appreciable. 

 The most important of these conditions are: (1) the character 

 and magnitude of the compression; (2) the temperature of 

 experiment in relation to the melting point of the substance ; 

 or perhaps more precisely, the amount of pressure required to 

 cause the substance to melt at the temperature of experiment. 



The effect of uniform (or even of nearly uniform) pressure 

 is usually comparatively slight ; for whereas uniform pressure 

 will always tend to bring about any reversible (enantiotropic) 

 change which is accompanied by a decrease of volume, it may 

 not — and very often does not — do so because the reaction 

 velocity under the specific conditions is not great enough. In 

 other words, pressure brings about such changes in all cases in 

 which the true equilibrium can readily be reached from either 

 side ; but if this is not so — either because the reaction velocity 

 is inappreciable or because the transformation is irreversible 

 (monotropic) — the condition produced is only an apparent 

 equilibrium. 



An exactly parallel case is the system hydrogen, oxygen and 

 water at the ordinary temperature. We know that the true 

 equilibrium in this system at ordinary temperatures is such that 



* Spring : Bull. Acad. Roy. Belg. (2), xlix, 351-2, 1880. 



t Rapport Congres Internat. de Physique, 1900. vol. i, p. 409. This 

 article is a resume of all of Spring's work up to that time. Of similar scope 

 is a lecture reprinted in Bull. Acad. Roy. Belg. (3), xxxvii. 790-815, 1899. A 

 resume of this work has also been given by C. F. Tolman, Jr. (J. Geology, 

 vi, 318-24, 1898 >, who, "however, does not discuss it critically, but merely 

 states the conclusions as given by Spring. 



