88 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



D 



A 



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of about 25 cm. The region under observation was limited by means of 

 a vertical diaphragm d, 5 mm. in width, wliicli was 



— i Tod, mounted in a tube in front of a window of the metal 



vacuum box. The comparison source was the spec- 

 trum of the brightest part of an acetylene flame set 

 d, up in the axis of the other collimator at a corre- 



sponding distance, and viewed through a circular 

 x/.. aperture c, 5 mm. in diameter, cut in a metal screen 



interposed between the flame and the slit and as 

 A near the former as practicable. 



The acetylene flame was adopted as a comparison 



standard for the fol- 

 (2, lowing reasons : — 



lAfinnn---^ l. it possesses 

 I " U U U u--''''^ a continuous spec- 

 trum, brighter in 

 the less refrangible regions than that of 

 anv other controllable source of liojit. 



2. Tlie radiating material is finely di- 

 vided carbon, presumably of a character 

 Figure 9. ^ot unlike that of the surface of the 



untreated rod. 



3. The acetylene fl ime is the result of the combustion of a definite 

 fuel (C2H2) burning under reasonably constant conditions. It is prefer- 

 able in this regard to any of the ordinary gas or candle flames in which 

 the fuel is of an undetermined and more or less variable character. 



4. When supplied with gas under constant pressure, an acetylene 

 flame of the type used in these experiments, that, namely, obtained by 

 means of a burner composed of a single block of steatite, is more nearly 

 constant in its intensity and color than any other flame with which I am 

 acquainted, with the exception of that of the Hefner lamp. It is indeed 

 questionable whether the latter is superior to acetylene in this respect, 

 and its comparative weakness in the blue and violet renders it very un- 

 desirable as a comparison source in spectrophotometry. 



Determination of the Temperature of the Acetylene Flame* 



Concerning tlie temperature of the acetylene flame, varying and in- 

 compatible statements are in existence. The temperature of combustion 



* The results of these experiments on tlie temperature of the comparison flame 

 were separately communicated to the American Physical Society ou February 24, 

 1900, and were published in the Physical Review, X. 234. 



