54 ENERGY CHANGES INVOLVED IN DILUTION OF AMALGAMS. 



ficial atmosphere of hydrogen, and responded to a change of one or two 

 thousandths of a degree. Needless to say, a powerful stirrer was used to 

 keep the bath in rapid circulation. 



The thermostat was allowed to run all night before a determination ; in 

 the morning the Beckmann thermometer was absolutely stationary at the 

 temperature of the bath outside. The partition P was pulled out by means 

 of the wire mentioned above ; the paddle, when thus released, revolved about 

 fifty times before the spring ran down. The temperature change was read 

 off on the Beckmann thermometer with careful tapping to avoid friction of 

 the thread. 



Unless mercury and amalgam are in perfect thermal equilibrium before 

 the experiment, the initial temperature, read from the single thermometer in 

 the amalgam, will not represent the average temperature of the reacting 

 mixture. Seventeen hours at constant temperature on all sides should in- 

 sure this condition for a comparatively small volume of a substance con- 

 ducting heat so well as mercury. Blank runs, however, were made with pure 

 mercury to test the truth of the assumption thus made. No cooling correc- 

 tion, of course, was necessary, though five minutes were allowed for the 

 thermometer to become constant. 



Blank experiment I : Temperature change -f- o.ooi . 



Blank experiment 2: Temperature change -f- 0.006 . 



This irregularity was found to be due to uneven heating of the calorimeter, 

 by radiation from the lamp used for heating the thermostat. This radiant 

 heat maintained one-half the mercury at a slightly higher temperature than 

 the other, on a cold night when the lamp was in operation most of the time. 

 On this account an electric heating coil 30 cm. in diameter was substituted 

 for the lamp ; its plane was horizontal, and the vessel F was exactly in its 

 center. Uneven heating could not now occur, and the following blank 

 experiments proved the change to have accomplished its object: 



Blank experiment 3: The thermometer fell 0.001 , and then rose to its 

 original position. 



Blank experiment 4: The thermometer rose 0.002 . 



The mean of these trials indicates that a correction of 0.00 1 in each 

 experiment should suffice for our purpose. This is to be ascribed to the work 

 done by the clockwork stirrer. 



A zinc amalgam (0.9 per cent) was now made from pure stick zinc and 

 mercury purified with mercurous nitrate. The manipulation of the two 

 determinations of the heat of dilution of this amalgam has already been 

 described. 



