in .Ferric Salts and in Chromic Acid. 323 



Tin.— The study of tin in chromic acid solution was confined 

 to the two experiments numbered 13 and 14 in the table, both 

 conducted in the presence of 5 molar sulphuric acid. The 

 results are in both cases apparently normal, and the reaction 

 velocity has the expected value. This behavior on the part of 

 a metal-forming soluble and stable salts of two different valen- 

 cies indicates one of two possibilities ; either (a) the second 

 stage of the oxidation does not occur to a measurable extent 

 (the case observed with tin in ferric sulphate), or, (b) the second 

 stage is so rapid that no appreciable diffusion of stannous salt 

 away from the metal can occur. As the reaction between 

 stannous sulphate and chromic acid is practically instantaneous 

 there can be little doubt that explanation (5) is the correct one 

 here. 



Silver. — Experiments with silver in chromic acid gave the 

 values recorded in N~os. 15-19 of Table X. In general, the 

 results resemble those given by silver in ferric sulphate, both 

 in the relatively low reaction velocities observed, and in the 

 fact that the velocity tends to decrease as the silver salt accu- 

 mulates in the solution. On account of this decrease the 

 reaction velocity characteristic of a given experiment is best 

 represented, as in a number of cases already considered, by the 

 " initial velocity," found by extrapolating back to time zero. 



GENERAL DISCUSSION OF RESULTS. 



A summary of the reaction velocities in chromic acid is given 

 in Table XL Comparison with the results obtained in ferric 



Table XI. 



Summary of Velocity Constants. 

 Chromic Acid. 

 Cone, of H.SO-4 0-25 1"25 5' molar 



Cadmium 7'02 5'32 2'67 



Nickel (irregular) (irregular) 2'67 



Tin .... 274 



Copper 6-95 5'3-f 272 



Silver 428 1-22 



sulphate, as summarized in Table VIII, shows a marked simi- 

 larity between the two series, although chromic acid always 

 gives higher velocities than ferric sulphate under like condi- 

 tions. The chief points of similarity are the following : 



(a) In each series there is an approximate agreement, in the 

 presence of 5 molar sulphuric acid, between the values for cad- 



