SIGNIFICANCE OF ''EQUILIBRIUM^' 161 



such cases equilibrmm^*' is not established during melting or solidifica- 

 tion, and no "point" can be found which possesses greater physical signifi- 

 cance than any neighboring point. To assign especial significance to any 

 single temperature is, therefore, wholly arbitrary. If the interval is 

 short, its limits can be determined approximately ; if long, there is little 

 of physical fact with which to determine them accurately. 



Significance of "Equilibrium" 



The uniform experience of the laboratory has been that where equi- 

 librium can be established during melting or solidification — that is, where 

 the molecular system is not too inert to react with reasonable promptness 

 to changes of temperature and pressure — there is a melting temperature 

 which is constant for the particular substance, which is absolutely charac- 

 teristic of that substance, and which can be determined. Such a melting 

 temperature is independent of the rate of heating and of other superim- 

 posed conditions. On the other hand, there are minerals (of which 

 quartz and albite were cited as examples) of such molecular inertness that 

 they do not reach equilibrium in the time available for a laboratory meas- 

 urement, and can not be made to do so unless the volatile ingredients 

 which must have participated in their natural formation can be restored. 



Effect of Pressure 



A number of experiments have already been undertaken in the labo- 

 ratory which serve to show that such a restoration is entirely practicable, 

 and which incidentally show the operating forces of nature's laboratory 

 in a more normal relation to each other. The field is not yet snfficiently 

 developed for more than a suggestion of the conditions which prevailed 

 in nature during the cooling of mineral masses containing volatile com- 

 ponents in solution, but the suggestion is worth noting. To the petrolo- 

 gist seeking to obtain exact information upon the behavior of his minerals 

 during heating and cooling, it has been somewhat puzzling to learn from 

 the laboratory that pure silica is not known to melt below 1600 degrees, 

 while his field observations clearly show that most observed natural quartz 

 must have crystallized considerably below 800 degrees. It has been equally 



^" By equilibrium is meant the situation in which all the operative forces so balance 

 each other that the system would remain indefinitely exactly as it stands, provided no 

 change is made in the pressure or temperature or composition. As soon as equilibrium 

 is established, physical measurements can be made with certainty and intelligently inter- 

 preted. Measurements made in transition periods under Indeterminate conditions are 

 very difficult to interpret at all. 



