1151 



by the fact (hat the high-meltinf!^ elements W, C, Mo, Pt, Ni, Fe, 

 when condensing on the glass-wall in vacuo ') produce a sublimate 

 that is either altogether optically non-soluble, or for a considerable 

 part optically non-soluble, and consisting for another part of distinctly 

 separate particles. The lower-melting elements with a greater vapour- 

 tension Au, Ag, Cu, Mg, show on the other hand a far greater 

 tendency to coarser condensation and form a connected network of 

 nltra-microns. 



Of the first group the highest-melting elements W, C and Pt and 

 also iron produce an optically insoluble precipitate. Of the larger 

 particles, which could then still be observed in the visual field, it 

 is uncertain whether they were metal particles and highly probable 

 that they were dust particles or impurities of the glass wall. The other 

 metals of this group — Mo and Ni — produced distinctly separate 

 particles. 



Of the metals of the second group Ag, Au and Cu offered a more or 

 less distinct network and in between also larger separate particles. 

 Mg, Zn and Cd showed on the glass wall at room temperature 

 local condensation and growth on account of the supersaturation of 

 the vapour, caused by refiection against the glass wall. The 

 condensate made to adhere to the glass-wall at a temperature of 

 liquid air, offered, when examined at room-temperafure, a coarse 

 heterogeneous appearance in which we could ascertain an arrangement 

 of the particles (which had probably set in during the heating at 

 room-temperature). 



So we can conclude: in proportion as the temperature of incan- 

 descence required for the slow sublimation, is higher, does the sublimate 

 formed assume a finer structure. 



The temperature of the glass-wall, on which the precipitate settles, 

 has a great influence on the nature of the same. Owing to a very 

 low temperature the deposits of Ag, Au and Cu become entirely 

 or at any rate for the greater part not optically soluble, whereas 

 Zn, Mg and Cd are obtained as uniform deposits, which ultra- 

 microscopically reveal the mosaic-structure. 



^) When not working with a sufficient vacuum and when the wall on which 

 the metal condenses is not brought to a sufficiently low temperature, the layers 

 will generally not be form'ed in a homogeneous state. With the numerous investi- 

 gations relating to constants of' thin layers of metal, the latter do not correspond 

 to the condition, in which those who examined them, imagined them to be. 

 Constants of homogeneous layers of metal, the thickness of which is slight with 

 regard to the wave-length of the light, have in our opinion not yet been satis- 

 factorily determined. 



