254 day: geophysical research 



by direct comparison with its natural prototype, the kind and 

 amount of effect actually produced in the natural mineral by the 

 one or more other minerals which it contains. We have therefore 

 hardly started upon our investigation before the need of an organ- 

 ized system is demonstrated, — first comes the chemist, who pre- 

 pares and analyzes the pure mineral for investigation; then the 

 physicist, who provides and measures the conditions to which it is 

 subjected; then the mineralogist, who establishes its optical prop- 

 erties in relation to the corresponding natural minerals. 



Having prepared such a mineral, of high purity and of known 

 crystalline character, we can ascertain its behavior at the temper- 

 atures which must have obtained during the various stages of 

 earth formation. We can study the various crystal forms thru 

 which it passes on heating and the temperature ranges within 

 which these forms are stable; we can also melt it and measure the 

 melting or solidifying temperature. Another mineral, prepared 

 with the same care and studied in the same way, may afterward 

 be added to the first, and the relation of these two determined. If 

 they combine, heat is absorbed or released; and this quantity 

 of heat can be measured, together with the exact temperature at 

 which the absorption or release takes place. If the mixture 

 results in the formation of one or more mineral compounds, we 

 shall learn the conditions of formation, the temperature region 

 within which the new forms are stable, and the changes which 

 each undergoes with changes of pressure and temperature, as 

 before. If the new forms show signs of instability, we can drop 

 them into cold water or mercury so quickly that there will be no 

 opportunity to return to initial stable forms, and thus obtain, 

 for study with the microscope at our leisure, every individual 

 phase of the process thru which the group of minerals has passed. 



Without complicating the illustration further, it is obvious that 

 we have it in our power to reproduce in detail the actual process 

 of rock formation within the earth, and to substitute measure- 

 ment where the geologist has been obliged to use inference; to 

 tabulate the whole history of the formation of a mineral or group 

 of minerals under every variety of condition which we may sup- 



