J 141 



of the sublimate could not be ascertained. With the precipitate foi-med 

 with — 185° the transition colourless-jellow-blne was somewhat more 

 raj)i(l than with the precipitate that arose with an ordinary' temperature. 

 On the whole the more i-eddish tints were absent. Allowing for the 

 experimental difficulties to provide foi* the filament a perfectly uniform 

 division of temperature, we must not set too much store by these 

 variations. 



§ 5. Gold. 



The first colour appearing with very slow sublimation in vacuum 

 is pink ; as the deposit becomes thicker, the colour becomes red, 

 violet, blue, then green, and finally the retlectioncoloui- of metallic- 

 gold is discerned. 



With access of air the pink tint gradually disappears and the 

 green becomes somewhat deeper tinted. But those changes are not 

 so strong as those observed with silvei'. Canadabalsam applied 

 to it in vacuum altogether checks it. 



Ultramicroscopically the difference between the parts covered- 

 and non-covered with Canadabalsam is not nearly so great as with 

 silver. The very thin, pink-coloured layer is optically insoluble, 

 the somewhat thicker deposits reveal a Tindalleffecl and the blue 

 and the green coloured ones show a more or less distinct mosaïc 

 of variously coloured ultramicrons, (cf. fig. 3. Plate I). 



On the whole the canadabalsam keeps the ultramicrons less luminous, 

 especially if the sublimation takes place very slowly and if during 

 that time the glass-wall is kept at a very low temperatnre. Fre- 

 quently it is only the Tindall-etfect with occasional strong particles, 

 which according to the image of a controllamp without gold, are 

 chiefly dust particles. 



Local heating — by keeping a hot glass-rod against the back of 

 the glass — effects, as with silver, a marked change. The green-blue 

 precipitate becomes red, with still intenser heating the centre becomes 

 yellow with a green i-eflex and red towards the edges, violet becomes 

 red and with intenser heating yellow. Generally the colour flows 

 back and the lightabsorption becomes slighter. Ulti'amici'oscopically 

 a very strong mosaic arose in the heated part. When pushing the 

 object under the microscope from the non-heated part to a heated 

 place, we see the mosaic appear more and more distinct and strong. 



In places where originally the precipitate was optically non-soluble', 

 the ultramicron-mosaic was brought to development by means of 

 the heating. 



Hence we observe with the gold as well as with the silver a 



79* 



