of the Metah. 



173 





A. 



H. 



w. 



9- 



Platinum 



76 51 

 70 48 



31 28 7 

 42 19 



1-9493 

 0-2705 



3-6097 

 2-531 



Gold 





a =45° 



Owing to the imperfections of the NicoPs prisms employed, 

 in which the incident and emergent rays were not parallel, the 

 values of A and H are affected by small errors. 



The transparent brown gold-leaf was prepared by spreading a 

 solution of gold, such as is employed for gilding in the porcelain 

 manufactories, on a piece of looking-glass and heating it in an 

 alcohol flame. In this instance, as also in the case of the platinum 

 films, the glass support was strongly heated and then badly 

 annealed. In order to eliminate the error arising from this, the 

 metallic films were detached from the glass plates by the employ- 

 ment of a method of E. Mitscherlich^s*, consisting in immersing 

 the plates in pure hydrochloric acid and then bringing them into 

 contact with pieces of zinc. The zinc with the other metal 

 formed a galvanic pair, and the hydrogen which was thereby dis- 

 engaged at the surface of the gold detached the latter from the 

 glass and it swam in the liquid. The liquid could then be 

 poured out with care, and replaced by distilled water. Having 

 washed the metal free from every trace of hydrochloric acid, 

 it was placed on a plane-parallel plate of polished glass, freed 

 from distilled water by evaporation, and finally lay flat on the 

 glass surface. By using a clean knife it was easy to give to the 

 film the requisite rectangular form. 



A determination of the thickness of a silver plate, and of the 

 displacement which it produced in the interference- bands, gave 

 the index of refraction 7i = 0'5. Since, however, in the determi- 

 nation of the thickness of the metallic plate the specific gravity 

 of the silver is assumed to be known, and it may vary, we must 

 look upon this number as an approximation only. 



However, the method above described does not admit of a 

 very accurate determination of the indices of refraction of metals, 

 since only very thin films of metal can be employed. This method, 

 however, is independent of the presence of holes in the metal ; 

 for these affect only the distinctness of the interference-bands. 



The displacement of the interference-bands might, however, 

 arise from air which is condensed at the surface of the metal 

 or the glass. Although a very great absorption of air at the sur- 

 face of solid bodies must be assumed in order to explain the 

 observed displacements of the interference-bands, nevertheless 



* E. Mitsckerlich, Lehrbuch der Chem. 2 Aufl. 1840, vol.ii. p. 340. 



