THE PROPERTIES OF METALLIC SUBSTANCES 401 



liar form of the conductance curve may be due primarily to the presence 

 of impurities. 



Many of these substances exhibit transition points at which the resist- 

 ance changes discontinuously. In some instances these processes are 

 reversible and in others irreversible. Silicon exhibits transition points 

 at approximately 220 and 440. Titanium exhibits discontinuities in 

 the neighborhood of 300 and 600, the first of which is slowly reversible 

 and the second irreversible. 



Among variable conductors are included many compounds on the 

 borderline between metallic and nonmetallic substances. These com- 

 pounds often appear in several modifications whose properties may differ 

 greatly. For example, silver sulphide, which has already been mentioned 

 in a preceding chapter, conducts electrolytically in one form, while in 

 another form it exhibits mixed electrolytic and metallic conduction. 

 Many solid oxides and mixtures of oxides, which at ordinary temper- 

 atures are nonmetallic, appear to conduct the current metallically at 

 high temperatures. The Nernst filament is a familiar example of this 

 type of conductor. While it is possible that a portion of the current in 

 some of these substances is carried electrolytically, the greater portion 

 appears to be carried metallically. 



As the compounds become more distinctively metallic, which is as a 

 rule the case as the more electronegative element becomes more metallic 

 and the more electropositive element becomes less metallic, the conduct- 

 ance approaches that of typical metallic compounds. In such cases, the 

 temperature coefficient becomes less negative or even positive. In gen- 

 eral, the higher the conductance of a compound, the greater is the value 

 of its positive temperature coefficient. 



Many of the conductors belonging to this class exhibit singular prop- 

 erties. In many cases, also, systems, which might not be expected to 

 exhibit metallic properties, nevertheless belong to this class of conductors. 

 Such is, for example, the case with cuprous iodide, Cul, which absorbs 

 iodine reversibly. The resulting product conducts the current metal- 

 lically and its conductance is the greater the greater the amount of iodine 

 absorbed. The smaller the resistance of the iodide, the greater is the 

 value of the positive temperature coefficient. As the specific resistance 

 increases, the temperature coefficient becomes negative. 



The examples of this class of substance are extremely numerous and 

 a great deal of experimental material is available. It is not to be doubted 

 that a study of such systems will throw a great deal of light on the nature 

 of the conduction process and conceivably on the constitution of metallic 



