410 



Mr. R. S. Willows on the Variation of the 



68°, there is a 



At Q, corresponding to a temperature ,, 

 sudden decrease in the resistance ; once this decrease has 

 commenced, no further increase in temperature is necessary 

 to complete it. If the temperature is very gradually raised 

 as the point Q is approached, and is kept constant by means 

 of a sensitive gas regulator at the temperature at which the 

 resistance begins to fall, then it is found that the fall takes 

 about an hour and a half to complete, while if the bath is 

 further heated for about 5° the change takes place in a few 

 minutes, in fact the length of time taken affords some test of 

 the accuracy with which the temperature at which the fall 

 begins is determined. 



The remaining part of the heating-curve is reversible on 

 cooling, i. e. the resistance is the same at the same tem- 

 perature, whether the body is being heated or cooled. 



The greater part of the cooling-curve is practically a 

 straight line, along which the resistance soon assumes its 

 final value when the bath is kept at a constant temperature ; 

 and, as the figure shows, the resistance is a great deal less at 

 a given temperature when the spiral is cooling than when it 

 is increasing in temperature. 



Along RN the resistance again takes some time to become 

 steady, and here it may alter considerably, although the 

 temperature, as given by a thermometer in the bath, has 

 remained constant. If along this part of the curve the bath 

 is cooled rapidly by adding cold oil, then, when the spiral is 

 allowed to stand, its resistance increases slightly for the first 

 twelve hours. The temperature corresponding to the mini- 

 mum resistance shown at R varies in the different amalgams 

 between 22°-26°, and does not seem to depend definitely on 

 the percentage of zinc present. 



On the other hand, the temperature corresponding to Q, 

 where there is a sudden fall in resistance, seems to decrease 

 more or less regularly as the amount of zinc present is in- 

 creased, as is shown in Table I. 



Table I. 



Percentage of Zinc. 



Temperature corres- 

 ponding to pt. Q,. 



Percentage fall in 

 resistance at Q. 



95 



74 



2-82 



161 



72 



915 



23-9 



68 



10-8 



28-3 



68 



129 



333 



59 



155 



40-1 



58 



15-9 



