370 Condensation and Reflexion of Gas Molecules. 



are fixed at the first impact. These, however, act as traps 

 and gather others about them, just as does a small area 

 of film. A granular deposit therefore forms, the grains 

 increasing in size until they are finally visible under the 

 microscope. 



In working with cadmium it was found that a film formed 

 in the bulb before the cadmium had reached the melting- 

 point. This film was black or neutral grey by transmitted 

 light. If, however, a piece of cadmium which had been 

 heated above the melting-point in vacuo for several minutes 

 was used over again in a fresh tube, the deposit was blue by 

 transmitted light, resembling a film of silver. Apparently 

 the cadmium contained some impurity which distilled off at 

 a comparatively low temperature. 



To determine the nature of this substance the deposit was 

 condensed in the bulb of a small end-on vacuum-tube with 

 external electrodes of tinfoil. 



The tube was sealed off from the cadmium tube, heated, 

 and the spectrum of the discharge photographed. The 

 cadmium and mercury lines were found on the plate. The 

 experiment was repeated with a long U-tube immersed in 

 liquid air between the pump and the tube, to prevent diffusion 

 of mercury vapour ; but again the black deposit was obtained 

 which showed the mercury lines. Evidently the cadmium 

 contains a trace of mercury, which distils off first, carrying- 

 some cadmium with it. 



The critical temperature for the alloy of cadmium and 

 mercury has not yet been determined. It is probable that we 

 are dealing with molecules of the alloy, for it has been found 

 that mercury vapour is capable of dissolving less volatile 

 metals, in the same way that the dense vapours of certain 

 organic compounds are capable of dissolving non-volatile 

 compounds, such as potassium iodide. 



Experiments were also made with iodine, for which the 

 critical temperature is in the neighbourhood of — 60°. Above 

 this temperature a black granular deposit forms, below it a 

 deep red film. The iodine crystals at one end of a tube bent 

 at two right angles were kept at 0°. The other end was then 

 plunged into a bath of known temperature, and the nature of 

 the deposit observed. When working in the vicinity of the 

 critical temperature, deposits were sometimes obtained which 

 were bright green by transmitted light. These probably 

 were made up of exceedingly minute granules, analogous 

 to the coloured films and fogs obtained with sodium and 

 potassium. 



