1146 



perfectly connected mosaic. Here again local heating tnrns the green 

 field into a red one and a copperreflecHon-colonr sets in. 



Another precipitate, formed by very slow sublimation (total 38 

 hours) and examined without the protection of canadabalsain, only 

 revealed the separate particles and no interlying mosaic. The separate 

 particles can but be a small part of the metal; hence the principal 

 part is optically insoluble. 



^13. Cachnium. 



This shows the phenomenon which R. Wood ^) has already pointed 

 out in a recent investigation, viz. that the vapour may be in an 

 extremely supeisaturated state and does not condense on the glass 

 wall. This appears very strikingly from the following experiments: 

 A cadmium lilamentlamp stands an electric current of 0.30 Ampère 

 without fusing off and without blackening. Then the current was 

 decreased to 0.26 Ampère and the wall of the bulb was locally 

 and momentarily brought into contact with wadding soaked into 

 liquid air. In this place a precipitate soon arises and it is only in 

 this place that the vapour-molecules further separate, so that after 

 some interval a fairly thick metallic reflecting layer, sharp-edged 

 with regard to the surrounding glass- wall, has formed. Finally the 

 filament is sundered. 



We conclude, that at first cadmiumvapour, developed in the 

 bulb, was in equilibrium with the electrically heated filament, but 

 greatly supersaturated as regards the cold wall. The vapourmolecules 

 striking against the glass wall were completely reflected. By local 

 strong refrigeration the oversaturation is neutralised. The vapour- 

 tension decreases and the equilibrium between the vapour and the 

 heated wire is broken. The metal now rapidly sublimates towards 

 the cold wall, until at last the filament becomes so thin, that it breaks. 



Not always was the over-saturation so strong. Sometimes a 

 condensate was formed spontaneously, even at room-temperature, but 

 it was very irregular as regards its thickness and in some places it 

 was altogether absent. 



With transmitted light the precipitate is blue, while somewhat thick 

 precipitates very soon reveal metallic reflections. A suchlike pieci- 

 pitate with metallic reflection showed in the ultramicroscope an 

 exceedingly strong mosaic of particles with great wealth of colour. 

 The apparent size of these particles does not show much difference 

 with that of silver, deposited at room-temperature. 



Another lamp that had been immersed in liquid air during the 



') Phil. Mag. (6) 32 (1916), 364. 



