WRIGHT: CHANGE IN ANGLES OF QUARTZ 485 



CRYSTALLOGRAPHY. The change in the crystal angles of 

 quartz with rise in temperature. Fred. E. Wright. Geo- 

 physical Laboratory. 



Crystallographers agree that in the development of the science 

 of crystallography the two minerals, quartz and calcite, have 

 played an exceedingly important role. Quartz was the first crys- 

 tal substance to attract the attention of the ancient Greeks. 

 They observed that its crystals are bounded by plane and not 

 curved surfaces, as is the case in plants and animals; and they 

 named it ''crystal," i.e., clear ice, on the assumption that it 

 had been formed from water thru the action of intense cold. 

 Later they found that other substances had different but equally 

 characteristic shapes and they accordingly extended the term 

 ''crystal" to signify the state of being bounded by flat surfaces 

 rather than to denote the mineral quartz to which the name 

 "rock crystal" is still applied occasionally. Further observa- 

 tions on crystals were not made until 1669, when Nicolaus Steno, 

 a Danish physician, found that^the angles between any two cor- 

 responding quartz crystals were the same even tho the shapes 

 and sizes of these faces varied from crystal to crystal. Steno 's 

 law of the constancy of crystal angles is of fundamental importance 

 and on it all subsequent work in crystallograpy has been based. 

 Steno's observations were made at room temperature and under 

 ordinary atmospheric pressure, and practically all subsequent 

 measurements of crystal angles have been made under similar 

 conditions of pressure and temperature, with the result that these 

 two factors have been less carefully considered by crystallogra- 

 phers than their importance possibly merits. 



We may look upon a crystal as a system of forces which finds 

 expression in the development of the crystal faces and in the 

 other crystallographic properties and which, in turn, exerts a 

 definite influence on external forces, either physical or chemical, 

 within the range of its action. This system is not invariant but 

 has two degrees of freedom, temperature and pressure, and, even 

 tho their effects may be relatively shght, yet their recognition and 

 study should furnish data of value. In physical chemistry the 

 investigation of the temperature-pressure effects on chemical- 



