TEMPERATURE COEFFICIENTS AND HEATS OF DILUTION. 5 1 



The agreement is surprisingly good; within the limit of accuracy of the 



measurement the increase of the potential with increase of temperature is 



identical with the increase of pressure of a perfect gas. A strong presump- 



dn 

 tion in favor of the constancy of jj^at all temperatures is established by 



these measurements. 



Time did not permit a careful study of the temperature coefficient of 

 pairs of zinc amalgams, but enough was done to show that the agreement 

 was almost as good in this case also. This matter will be discussed pres- 

 ently from another point of view. 



THE MEASUREMENT OF THE HEAT OF DILUTION OF THE AMALGAMS. 



The next important step in the experimental work was to determine 

 directly the heat of dilution, in order to apply rigorously not only the equa- 

 tion of Helmholtz, 



vn F U = vF^ 

 a 1 



but also the equation of Cady and of Lewis, 



virF U = vFln ^ 



and thus to throw light on the cause of the deviations noted in the curves 

 discussed in the previous sections. 



The great consistency and accuracy of the preceding electrical measure- 

 ments makes it highly desirable that the heat of dilution, which in this case 

 is the heat of reaction of the system under investigation, should be deter- 

 mined with great precision. In order to show the grade of accuracy needed, 

 attention is called to the fact that each hundred thousandth of a volt in the 

 potential of a concentration cell corresponds to the development of two joules 

 during the transport of a gram atom. Consider now a gram atom of zinc 

 contained in 7 kg. of amalgam 3 in the act of dilution with an equal volume 

 of mercury. The heat capacity of the reacting mixture will not be far from 

 2,000 mayers ; M hence two joules will produce in it a temperature change of 

 one-thousandth of a degree. If the thermometer can be read as accurately 

 as this, the accuracy reached in the work on potentials will be maintained. 

 The same dilution of a gram atom of cadmium in cadmium amalgam No. 1 

 will involve a heat capacity only about half as great; here, therefore, the 

 thermometer will be read twice as accurately as the potentiometer. If the 

 amalgam resulting from either of the above reactions is diluted with its own 



A mayer is the heat capacity raised i C. by a joule. 



