1140 



Consequently the silver deposit consists of two parts: 1«* a number 

 of more or less perceptible ultramicrons and 2"'^ an optically insoluble 

 pari, which like the layer with NaCl, CaF, and W can be looked 

 upon as vitreous-amorphous. Owing to the heating the second part 

 merges into the first. 



We see that the absorption-colour of the precipitate, as observed 

 with the naked eye, changes during its growth in the direction 

 yellow-red-blue, whereas during heating it is modified in an opposite 

 direction. Hence it appears that this absorption-colour is chiefly 

 determined by the vitreous amorphous part and not by the ultramicrons. 



The lowest temperature, in which these changes set in was now 

 more closely investigated. 



Knudsen ^) states, that as regards the appearance of the silver 

 sublimate no difference could be observed whether the temperature- 

 of the glass-wall against which it is deposited, is 575° or several 

 hundred degrees lower. 



But we already saw, that heating to 260° caused an appreciably 

 difference in the layer formed with 20°. 



Heating to ISO'' had a similar effect. The blue tint changed into 

 green-yellow. 



With a rise of temperature from 20° to 80° and a short duration 

 of heating we could not observe any change. It is probable, tliough, 

 that in time a change will set in, so that a sharp demarcation-line 

 above which the deposit is unstable and below which it is stable 

 cannot be drawn. 



We tried to investigate whether the sublimates formed under a 

 very low temperature differ at all from those arising under room 

 temperature. With former processes concerning this matter the result 

 attained had been but a negative one; with both temperatures the 

 same colour-scale was gone through. It could be expected that eventual 

 variations would be but very slight ones. In order to be able to 

 ascertain this, we chose a cylindrical glass-balloon with a straight, 

 axial filament, being immersed midway in liquid air during the time 

 of burning. Hence the lower part of the glass had a temperature 

 of about — 180°, the upper part was at room temperature. As, owing 

 to the great heat-conducti\'ity of silver, a strong refrigeration takes 

 place with the leadiug-in and suspensionwires, the filament must be 

 fairly long in order to glow equally along a sufficient distance. In 

 our case the length of the double-filament was 20 and the diameter 

 of the glass-cylinder 5 cm. A very distinct difference in the colour 



1) Ann. d. Physik. (4) 50 (1016), 472. 



