Intelligence and Miscellaneous Articles. 243 



on all sides except through a small peep-hole; the following 

 is then observed. At the first beginning of the red heat the 

 porcelain begius to be luminous. The black spot stands out 

 from it. As the temperature rises the emission of light by 

 the porcelain becomes more intense, and the whole contents of 

 the muffle seems as if it had been feebly illuminated from the 

 outside. If the heat be still more increased the black spot 

 becomes feebler, and after passing through a relatively small 

 interval of temperature, it stands out so little from the por- 

 celain that anj r one seeing it for the first time would think it had 

 completely disappeared. When a burning spill or a gas-flame is 

 brought into the muffle, it is seen that the spot is still black on a 

 white ground (the black having a dark red rusty tinge). This 

 phenomenon, which might be used for recognizing distinct tempe- 

 ratures, I estimate to occur at 800° C. If the temperature be now 

 raised, the emission of light of the dark spot precedes that of the 

 porcelain, and at about 1000° to 1100° 0. it appears bright, radia- 

 ting white on the rose-red porcelain. By introducing a burning 

 body into the muffle it again appears dark on white. Other por- 

 celain colours, purple for instance, give similar phenomena; a 

 pen-stroke (which changes into oxide of iron and burns in with 

 lustre) is sufficient ; but no material gives so intense a colour, and 

 is so little interfered with by reflexion, as that mentioned. 



The phenomenon is simply explained. Porcelain, at ordinary 

 and even higher temperatures, is transparent for luminous rays; 

 the dark mixture of metallic oxides, however, is opaque, as can 

 easily be seen with a painted porcelain crucible made red hot in the 

 gas-flame. In the measure in which the luminous rays increase in 

 intensity with increase of temperature, the emission of light by the 

 black spot also increases. As it always appears black when illu- 

 minated with a source of high temperature, it follows that it must 

 have a greater power of absorption than porcelain ; that is, if the 

 heat were increased its brightness would also increase, provided 

 there were not otherwise any change in the power of absorption. 



The experiment may of course also be made in a darkened room 

 with a piece of porcelain, and forms thus an instructive lecture- 

 experiment. I find that it succeeds pretty well if a large porcelain 

 crucible is painted on the inside and heated in a Bunsen's burner. 

 The disappearance of the black, however, does not succeed very 

 well ; and by free heating in the flame we cannot get a temperature 

 of more than a dark redness. On the other hand, with a spot of 

 gold an intensely green radiation is obtained at about 800° (like 

 the surface of melted copper), which at decreasing temperature 

 passes into a dark blue. The colours recall the colours of trans- 

 mission of thin gold leaves. Platinum is far more luminous on 

 cooling than porcelain; the light disappears, passing through a 

 faint red, like that of the other opaque solids. The behaviour of 

 gold and platinum shows distinctly that certain kinds of rays have 

 a specific emission. — "Wiedemann's Annalen, No. 2, 1888. 



