560 Mr. W. Kosenhain. Further Observations on [Feb. 9, 



prepared surface, while the arrow C indicates the proper direction of 

 rubbing during the final polishing process.* 



The film of metal smeared over the boundary under these circum- 

 stances is extremely thin,t and can be removed by slight etching, 

 preferably by means of a reagent which dissolves the iron while 

 leaving the copper untouched. Such a reagent is found in a 10-per- 

 cent, solution of picric acid in alcohol, acting for about 30 seconds. 

 This treatment leaves a clearly defined boundary line appearing under 

 a certain incidence of " vertical " illumination as a narrow black line, 

 and under other illumination being visible merely by the colour- 

 contrast between the iron and copper. Even under the highest power 

 of the microscope the boundary line can be seen sharply in focus, but 

 it then becomes evident that there is always a very slight difference 

 of level between copper and iron, and that the black line seen at the 

 boundary under certain conditions of illumination is the shadow of 

 the edge of the copper projected on the iron. As the copper is 

 entirely unacted upon by the etching reagent, the edge of the copper 

 may be taken as the true outline of the transverse section of the 

 previously prepared surface. 



When a previously polished and etched specimen of iron which has 

 had slip-bands developed upon its surface by strain is treated and 

 examined in this way, the boundary line shows well-marked steps or 

 serrations, readily visible under a magnification of 1000 diameters. 

 It was, however, necessary to show that these were really the sections 

 of slip-bands, and were not due to any of the other processes gone 

 through by the specimen, such as the initial etching of the prepared 

 surface or the electro-deposition itself. For this purpose a series of test 

 specimens were prepared and treated in a similar manner. The first 

 of these had been polished in the initial stage, but neither etched nor 

 strained, and the boundary in the transverse section showed an almost 

 unbroken straight line, thus proving that the electro-deposition in no 

 way produced irregularities on the boundary ; in fact, the extreme 

 closeness of adhesion of the deposit, and the accurate manner in which 

 it follows the minutest details of the prepared surface, is a most 

 remarkable feature of the whole process. 



It was at first thought that the examination of such boundaries 

 might throw light on the mode of adhesion of electrolytic deposits, 

 but none of the features observed afford arfy evidence on that 

 question. In a second specimen the initial surface had been polished 

 and etched, but not strained, and here also there was an entire absence 



* The special process of polishing here described was worked out in conjunction 

 with Mrs. W. Rosenhain, who entirely carried out its application. 



f Mr. Beilby believes such surface films of metal to be transparent, but in the 

 present instance, where one metal was smeared over the other, transparency would 

 have been easily detected, but no evidence of its existence was found. 



