52 ENERGY CHANGES INVOLVED IN DILUTION OF AMALGAMS. 



volume of mercury, the heat developed by the reaction is distributed over a 

 heat capacity double that noted above. Then the thermometer must be read 

 twice as accurately as before estimated, in order to compensate for this fact. 



Extending this train of reasoning, it is evident that excessive refinement 

 in temperature measurement would be required to obtain a good experi- 

 mental value for the heat of reaction of more dilute amalgams, where the 

 heat capacity per gram molecule of zinc is enormous. Limitations in time 

 made it necessary to confine our attention to concentrated amalgams. In all 

 probability the generalizations established for these cases will apply at greater 

 dilutions also. 



The measurement of the heats of dilution presents considerable experi- 

 mental difficulties. Practical considerations make it desirable to limit the 

 weight of mercury used in one experiment to 5 kg. ; but the small heat 

 capacity of such a mass and its rapid conduction of heat demand extraordi- 

 nary constancy in the temperature of the surroundings. Owing to the high 

 inertia of mercury and its low heat capacity, efficient stirring of the mixture 

 is likely to produce a marked rise in temperature ; this must be estimated 

 and the corresponding correction applied. Moreover, the entire process must 

 be carried out in an indifferent gas, to prevent oxidation, with its attending 

 development of heat. 



To meet these requirements a new form of calorimeter was devised. The 

 principle of the divided vessel was suggested by the work of Richards and 

 Lamb," but it is believed that other features of the apparatus constitute a 

 new departure in the study of small heats of reaction. 



The calorimeter C is shown in figure 9. It was made of flexible thin 

 iron plate seamed together. The partition P slid in deep grooves, b and b 1 , 

 while its lower edge was received by a shallow trough of sheet iron cemented 

 to the bottom of the calorimeter ; when P is in this position its top is flush 

 with the rim of the vessel. A grease containing about one part of gum 

 rubber, five of hard paraffin, and ten of soft paraffin applied around the edge 

 of the partition made either side of the vessel mercury-tight. When the 

 ebonite support R had been attached, C was lowered into the copper cylin- 

 der F, leaving an air space 0.7 cm. in thickness. 



A weighed amount of pure mercury was now run into one side of C from 

 a separating funnel. Then the clockwork stirring apparatus was lowered 

 into F and bolted rigidly to brass ledges soldered to the inner wall of F. 

 The mechanism consisted merely of the spring, its arbor, and one gear, whose 

 shaft carried the paddle D. The latter now dipped into the mercury, but its 

 blades lay flat against the partition P ; therefore, when the spring was wound 

 up the paddle could not revolve until P was pulled out. 



Richards and Lamb, Proc. Am. Acad., 40, 657 (1905)- 



