On the Temperature of Incandescence. 223 



of a Bunsen burner, it could not be rendered incandescent, 

 though the platinum wire glowed brightly. Examination with 

 a spectroscope showed that the silica thread gave practically no 

 spectrum, and consequently emitted no light whatever, except 

 at points where some opaque foreign particle might be entangled 

 within it. Bearing as it does upon several applications of quartz 

 in the form of lenses and prisms, and upon various diathermic 

 considerations, this experiment evidently suggests the desira- 

 bility of an examination of the relation to radiant energy of 

 quartz which has been fused and which can therefore no longer 

 contain microscopic water-cavities. Professor C. A. Young, who 

 saw it at a meeting of the American Astronomical Society, in 

 June, 1883, regarded it as a very pretty experiment and probably 

 a consequence of Kirchoff s law. 



As is usually the case when once a general principle is revealed 

 to us, evidences of it are easily recognized in the most familiar 

 instances, and the phenomena of this principle are presented to 

 us in all varieties by the ordinary blow-pipe beads. 



If a bead of borax or microcosmic salt be heated and allowed 

 to cool, the bead itself at any moderate temperature is hardly 

 visible ; but even after it has cooled in a dark room until quite 

 invisible, the platinum wire ring still remains red-hot, and may 

 be distinctly seen. A bead of carbonate of soda behaves differ- 

 ently. When very hot, in a dark room it appears almost non- 

 luminous, even though the ring of platinum wire glows quite 

 hot, but at the moment of solidifying it suddenly incandesces 

 with a red heat, and it and the wire then degrade together in 

 luminosity until they become, at the same instant, invisible. 



Upon the surface of, or within such a bead, of borax or car- 

 bonate of soda, while hot and transparent, a foreign particle 

 floats about red-hot ; and under a considerable depth of certain 

 melted fluxes, I have seen a button of pure silver, at the bottom 

 of a crucible, almost as clearly as it could be seen through so 

 much cold glass. 



Another interesting instance involving this principle occurs in 

 the case of melted gold, which is less luminous at a certain tem- 

 perature, than after it has cooled down to the point at which it 

 solidifies, when it suddenly emits a brilliant light. 



