BAD CONDUCTORS. DIELECTRICS. 409 



According to Poiseuille, the internal friction of water is inversely 

 as the expression 



i +0*0336793 t+ 0*000209938 / 2 . 



It follows accordingly that the variations with the temperature of 

 the electrical conductivity of a very dilute saline solution depend 

 simply on the internal friction of water. 



The law of Grossmann applies also to fused salts, as follows from 

 the experiments of Foussereau. The constancy in the value of the 

 product of the conductivity into the coefficient of internal friction 

 differs with different salts, and these different products do not seem 

 to offer any simple ratio, either to each of the coefficients or to the 

 chemical equivalent of the salt. 



1005. BAD CONDUCTORS. DIELECTRICS. Certain bodies, such 

 as graphite, selenium, and various metallic sulphides, behave like 

 metals, although their conductivity is much less ; but their resistance 

 diminishes as the temperature rises. The specific resistance of the 

 carbons used for the electric light is about 0*004 ohm, or 42 times 

 that of mercury; it varies about 0*0003 of a degree between the 

 temperature of zero and 100. 



The resistance of selenium depends greatly on its structure ; 

 the metallic form conducts far better than the crystallised form. 

 Mercadier* finds that the resistance of selenium decreases con- 

 tinuously as the temperature rises from o to 125, and that then, 

 towards 163, it has a relative maximum corresponding to a change 

 of state. Between zero and 36 the change is very rapid, and is 

 sensibly proportional to the change of temperature. According to 

 Shelford Bidwell, on the contrary, the resistance passes through a 

 maximum between 20 and 30. 



Selenium has, moreover, the curious property, observed first by 

 Willoughby Smith, of becoming a better conductor under the action 

 of light. The increase of conductivity by illumination disappears 

 with extreme rapidity when the selenium is kept dark. 



Phosphorus, and particularly sulphur, present a great resistance 

 when solid, but become relatively conductors when they are 

 melted. Their conductivity increases rapidly with the temperature. 

 Foussereau found that the resistance of these two bodies could be 

 represented by an expression of the form 



(57) log. R^a 



* Comptes rendus, Vol. xcn., p. 1407. 1881. 



