THE PROPERTIES OF METALLIC SUBSTANCES 393 



small, while the resistance-temperature coefficient is large; consequently, 

 the temperature coefficient is not greatly affected by the volume change. 

 In liquid metals, however, where the temperature coefficient is small and 

 the resistance pressure coefficient relatively large, the volume change has 

 a material influence on the observed temperature coefficient. In the case of 



mercury, 11 the resistance temperature coefficient I D ) 1 -JT I 6.9 X 10' 4 



as against the value of + 8.9 X 10~ 4 for the resistance-temperature coeffi- 

 cient at constant pressure. It is a significant fact that the resistance of 

 a liquid metal at constant volume should decrease with increasing tem- 

 perature. In this connection it may be noted that Somerville 12 -found 

 that zinc wire, wrapped in the form of a spiral around a silica tube, 

 exhibited a marked negative temperature coefficient above the melting 

 point, the resistance varying very nearly as a linear function of the tem- 

 perature. In this case the metal in the fluid state was held together by 

 surface forces. The temperature coefficient of molten zinc in a quartz 

 tube was found to be positive but of a very low value. 



5. The Conductance of Metallic Alloys. Metallic alloys may be 

 divided into four classes which exhibit distinct properties. 13 These are: 

 First, solid alloys in which pure crystals of the constituent elements are 

 present in intimate contact; second, solid alloys in which mixed crystals 

 of the constituent elements are present; third, solid alloys in which com- 

 pounds of the constituent elements are present; and fourth, liquid alloys. 

 Among the solid alloys, several of these types often appear in a single 

 alloy. This is the case, for example, when mixed crystals are formed 

 over limited concentration intervals. 



a. Heterogeneous Alloys. Except in so far as the resistance of alloys 

 is influenced by the distribution of the crystals and the presence of resist- 

 ance at the interface between crystal elements, solid alloys of the first 

 class do not differ in their properties from pure metals. The specific 

 resistance of such" alloys is a linear function of the composition and with 

 change of temperature the properties vary as a linear function of the 

 composition, 



b. Homogeneous Alloys. Homogeneous mixed crystals of pure 

 metallic elements, or of compounds, form an important class of substances 

 which are remarkable for the uniformity of their behavior among them- 

 selves and the divergence of their behavior from that of their constituent 

 elements. The addition of a second metallic component to another metal, 



"Kraus, Physical Review k, 159 (1914). 

 12 Somerville, Physical Review 33, 77 (1911). 



u A summary of the properties of metallic alloys is given by Guertler, Jahrb. /. Rad. 5, 

 IT 



