in Ferric Salts and in Chromic Acid. 331 



(c) Passivity. 



(d) Evolution of a gas. 



These four types seem to include all that have thus fat" been 

 reported.* Of these, passivity is indicated by a marked depend- 

 ence of the initial reaction velocity upon the preliminary treat- 

 ment of the metal, and a two-stage reaction can be detected 

 either by analysis of the solution or by the abnormally small 

 effect of variation in the rate of stirring upon the observed 

 reaction velocity. f Insoluble or gaseous products will usually 

 show their presence by visible effects. All of these types of 

 interference are of such a nature that they could generally be 

 predicted in advance. 



Summary. 



Measurements have been made of the rates at which different 

 metals react with the same oxidizing solution in the presence 

 of varying concentrations of free acid. The solutions used 

 were (a) ferric sulphate and sulphuric acid, (o) ferric chloride 

 and hydrochloric acid, and (c) chromic acid and sulphuric acid. 

 The chief experimental results are the following : 



1. When the acidity is sufficiently high, a number of metals 

 give the same reaction velocity under like conditions, showing 

 that diffusion is here the determining factor. 



2. With decreasing acidity such agreement tends to disap- 

 pear, and the observed velocities then depend upon the nature 

 of the metal, the order being approximately the same as the 

 electromotive series and the velocity greater the more electro- 

 positive the metal. 



3. The rate of the reaction in normal cases is proportional 

 to the concentration of the oxidizing agent, not only under 

 conditions where different metals give the same rate, but also 

 where the rate depends upon the specific nature of the metal. 



From these and related facts the following conclusions are 

 drawn. They apply only to normal cases, that is, to those in 

 which the progress of the reaction is not interfered with by 

 mechanical effects, such as insoluble coatings and the like. 



* Nernst, in his " Theoretical Chemistry," 3d English Ed., p. 587. specifi- 

 cally mentions only the first two. 



f An interesting case of a two-stage reaction is that of arsenic trioxide 

 when dissolving in water. After dissolving as As 2 3 it undergoes hydration 

 either in the solution or, possibly, as E. Brunner has suggested, wholly in 

 the diffusion layer. This second stage is of course not detectable by chem- 

 ical analysis. See Drucker, Zeitsehr. phys. Chem., xxxvi, 201 and 698, 1901; 

 L. Bruner and Tolloczko, Zeitsehr. anorg. Chem., xxxvii, 455, 1903 ; E. 

 Brunner, Zeitsehr. phys. Chem., li, 494, 1905. 



