374 Dr. Karl Heumann's Contributions to 



a continuous spectrum. If the metal or oxide which is sepa- 

 rated in the flame be partially volatilized, one part of it may 

 remain in the solid form and so impart luminosity to the flame, 

 while another portion may become gaseous and so impart 

 colour to the flame. The luminous portions of such flames 

 show a continuous, the coloured portions a line-spectrum. 



Those substances which do not cause a non-luminous flame 

 to become luminous may also be subdivided into those (salts 

 of lead, of alkalies, alkaline earths, except magnesia, &c.) 

 which are easily volatilized and therefore colour the flame 

 while affording line-spectra, those (arsenic and antimony com- 

 pounds) which afford continuous spectra while also imparting 

 a colour to the flame, those (mercuric chloride) which vola- 

 tilize almost immediately without producing any noticeable 

 effect upon the flame, and, lastly, those (magnesium and 

 silver chlorides) which, although undergoing decomposition 

 with the production of solid matter, nevertheless do not cause 

 luminosity because the solids produced are not carried into the 

 flame. The metal or oxide which is separated in the various 

 experiments may be obtained by bringing a porcelain basin 

 into the flame. 



If it be true that the luminosity is due in the foregoing ex- 

 periments to solid matter separated within the flame while the 

 colour is caused by heated vapours, we should expect to find 

 the luminous flames casting shadows upon a white background 

 when viewed in sunlight, while the coloured flames would cast 

 no shdaows. 



Experiment has proved the correctness of this expectation in 

 every case. 



The flames of burning magnesium and of coal-gas contain- 

 ing oxygen and metallic zinc both cast very distinct shadows ; 

 these flames contain magnesium and zinc oxides respectively, 

 substances which remain solid at high temperatures. 



From these experiments I think we may draw the follow- 

 ing inference : — 



Luminous flames which oive their luminosity to the presence 

 of finely divided solid matter produce characteristic shadows when 

 viewed in sunlight. 



But is the converse of this true ? Do luminous flames 

 which produce shadows in sunlight owe their luminosity to 

 the presence of solid matter ? 



To this question, in the absence of experimental evidence, 

 I should answer no ; for it is possible that luminous flames 

 consisting only of heated gases may, when viewed in sun- 

 light, cause an appearance similar to that of a true shadow. 

 We know that the electric light, when passed through 



