in Ferric Salts and in Chromic Acid. 311 



tions of the solutions were only approximate, and the potential 

 readings changed somewhat with the time, but the effect of 

 the added salts is unmistakable. It is also clear that the elec- 

 trode in all cases becomes less positive or more negative to the 

 solution as the acidity increases, a fact to which we shall have 

 occasion to refer later. 



The reaction between ferric ion and a metal would, in 

 general, be expected to be more complete the less noble the 

 metal, and this probably explains why the tendency of the 

 reaction velocity to decrease during the course of the experi- 

 ment, though large enough to affect the results in the case of 

 silver, and (to a smaller extent) also in the case of copper, was 

 not appreciable with tin, nickel, iron, and cadmium. 



On account of this decrease in the constants it is clear that 

 the extrapolated initial velocities, in all the silver and copper 

 experiments, are the values which properly represent the 

 individual experiments. 



Zinc. — For the experiments with zinc the metal used was 

 the very pure commercial grade from the "Bertha" mine in 

 Pulaski, Virginia. This was cast into thin plates and then 

 ground and filed to the desired shape and thickness. Un- 

 fortunately it proved impossible to obtain the rate of solution 

 of zinc, free from the disturbing influence of an evolution of 

 hydrogen, which was given off in appreciable quantity even in 

 ferric sulphate solutions to which no acid had been added. It 

 has been shown in a previous paper* that the evolution of a gas 

 tends to raise the observed reaction velocity, and by an uncer- 

 tain amount, so that the velocity constants obtained in Experi- 

 ments 14 and 15, in the presence of O'Ol molar sulphuric acid, 

 are probably slightly too high. 



These two experiments are the only ones recorded in this 

 paper which are affected by this source of error. None of the 

 other metals gave an appreciable evolution of hydrogen under 

 the conditions of experiment. 



Rates of Solution in Ferric Chloride. 



No change in the conditions or general procedure 

 described above for ferric sulphate was made in the experi- 

 ments with ferric chloride, except that in making the titrations 

 definite amounts of manganous sulphate, sulphuric acid, and 

 phosphoric acid were added to the sample, instead of phosphoric 

 acid alone. This modification, made necessary by the presence 

 of chloride, involved a slight sacrifice in the accuracy of the 

 analyses. Table VII contains the results. 



The experiments with cadmium call for no special comment. 

 In the experiments with metallic iron the disks remained 

 *This Journal, xxxii, 217. See also, ibid., xxxvi, 544. 



