SCIENCE AND PRACTICE. 273 



for some hours, adding sulphuric acid when necessary, and, 

 from time to time, a few drops of nitric acid. Dr. Mat- 

 thiessen allows his mercury to stand some weeks under 

 a cover of dilute nitric acid, with which it is frequently 

 agitated. 



Another recommendation in favour of mercury is the 

 fact that, being fluid, its molecular condition cannot be 

 subject to changes, as may be the case with the solid metals, 

 especially in the process of drawing, and that, so long as 

 it is pure, it must always have the same conducting power 

 at the same temperature. 



Lastly, the variation of the conducting power of mercury 

 with difference of temperature is considerably less than that 

 of the pure metals in a solid state. The amount of this 

 variation, as determined by Becquerel, for all temperature 

 degrees between and 100 C., may be taken, without 

 sensible error, as directly proportional to the difference of 

 temperature, being equal to the 0,00104th part of the con- 

 ducting power for each degree of the centigrade scale. A 

 more recent determination by Dr. Matthiessen, which has 

 been equated by the method of least squares, with two 

 members, taking the conducting power of mercury at C. 

 as = 1, gives the conducting power C t , at the temperature 

 t C. 



C, = 1 0,00074432* 0,0000008261 P. 



The conducting power of mercury alloyed with even 

 a very minute quantity of foreign metal is greater than 

 that of pure mercury; hence great care is necessary, in 

 using this metal, to procure it as free from impurities as 

 possible. 



35. Electric Permanency of Metals. The conducting powers 

 of some of the metals in a solid state at the same tempera- 

 ture seem to be pretty permanent; others, again, appear 

 to alter their conducting powers materially in the course 

 of a few months. As a rule, the hard-drawn wires are the 

 least permanent, by reason of their becoming gradually 

 annealed by exposure to variations of temperature. Dr. Mat- 



T 



