1143 



weaker network, Tindalleffect, extremely weak or no TindallefFect 

 at all. 



The network is not always uniform in the strongly-heated parts. 

 There are sometimes more and less luminous sections, while occasi- 

 onally the ultramicrons no longer coalesce, but are broken up into 

 distinctly separate larger particles. This partly depends on the local 

 thickness of the Pt-layer, partly also on the fact that the glass- 

 wall has been heated to a state of softening. 



^ 7. Tungste7i. 



The precipitate formed at room temperature is altogether optically 

 insoluble and uncommonly proof against a change in temperature. 

 If the vacuum is well-maintained, heating up to 380° does not 

 effect a microscopically perceptible change and ultramicroscopically 

 at the utmost a weak Tindall-effect is observed. Distinct conglome- 

 ration only sets in when the glass is heated up to its melting-point 

 by means of a thin flame. If instead of the easily melting lead-glass, 

 we used a balloon of ordinary high-fusible glass, we observed 

 that in this case as well, an appreciable lightning of the ultra- 

 microscopic appearence only set in, when the glass had been 

 heated to softening. So the cause of the conglomeration must not 

 be looked for in the greater mobility of the tungsten particles, 

 but in a shifting of the subsoil. Hence the ultramicroscopic figure 

 was quite different from that with Au and Ag. Instead of a regular 

 mosaic we see a conglomeration with fibrous structure, that some- 

 what reminds us of a wrinkled-up film. 



§ 8. Molybdenum. 



As with W and Pt it forms a colourless precipitate from greyish 

 to black, Ultramicroscopically we see a great many but slightly 

 luminous separate particles, which have a tendency to unite into 

 small wreaths of 10 or 20 or into longer rows. Especially when the 

 precipitate is not protected by Canadabalsam these groupings 

 become very marked; but when the precipitate, by means of careful 

 sublimation is formed in a high vacuum, being .then covered w^ith 

 CaFj and Canadabalsam, or Canadabalsam only, they occur as well. 

 A connected network between the separate particles is not to be 

 seqn. 



A similar image became visible, when during the entire time of 

 burning the lamp was immersed in liquid air. 



If a lamp in which a very dark precipitate had formed, was 

 heated to 380° — 400° when still in vacuum, being then covered 



