414 MR WILLIAM SWAN ON THE PRISMATIC SPECTRA OF THE 



to tinge a flame with bright yellow light ; and as the equivalent weights of sodium 

 and chlorine are 23 and 35'5, it follows, that a quantity of sodium not exceeding 

 2506066 °f a * r °y S ram renders its presence in a flame sensible. If it were possible 

 to obtain a flame free of yellow light, independently of that caused by the salt 

 introduced in the experiment, it is obvious that a greatly more minute portion of 

 sodium could be shown to alter appreciably the colour of the flame. It therefore 

 follows, that much caution is necessary in referring the phenomena of the spectrum 

 of a flame to the chemical constitution of the body undergoing combustion. For 

 the brightest line in the spectrum of the flame of a candle, — the yellow line R* of 

 Fraunhofer, — can be produced in great brilliancy, by placing an excessively 

 small portion of salt in a flame, in whose spectrum that line is faint or alto- 

 gether absent. The question then arises, whether this line in the candle flame 

 is due to the combustion of the carbon and hydrogen of which tallow is chiefly 

 composed, or is caused by the minute traces of chloride of sodium contained in 

 most animal matter. When indeed we consider the almost universal diffusion of 

 the salts of sodium, and the remarkable energy with which they produce yellow 

 light, it seems highly probable that the yellow line R, which appears in the 

 spectra of almost all flames, is in every case due to the presence of minute 

 quantities of sodium. 



The view, which would attribute a great portion of the light of the envelopes 

 of flames to the adventitious presence of minute traces of foreign matter, may 

 possibly serve to explain certain anomalous diversities of colour which are 

 observed in the envelopes of flames arising from the combustion of the same ele- 

 ments. Thus tallow, coal gas, anhydrous alcohol, and weak spirit of wine, all 

 contain the same combustible substances, carbon and hydrogen : yet the envelope 

 of the flame of a candle is bright yellow, that of a coal gas flame is purple, and 

 those of strong alcohol and weak spirit differ greatly in luminosity. 



It is important also to remark, that while the luminosity communicated to 

 the exterior envelope of a flame by such substances as the salts of sodium or of 

 copper, may be so great as to disguise that of the inner bright cone of the flame, 

 or in some cases to render it altogether invisible ; yet I have ascertained that the 

 light of the blue portion of the flame, or of the inner cone, remains absolutely 

 unchanged in colour and intensity. The proof of this curious property of flame 

 will be given in the sequel, f 



Prismatic Analysis of Flame. 

 Reserving, meantime, a more complete description of the apparatus I have 

 employed, it may be sufficient to premise, that, in what I shall have to say regard- 

 ing the spectra of flames, the object observed is supposed to be a narrow illumi- 

 nated slit, viewed through a glass prism mounted before a telescope, which has 

 been adjusted to focus on the slit. 



* Schumacher's Astronomische Abhundlungen, 1823, p. 18. f See p. 419. 



