88 



PROCEEDINGS OP THE AMERICAN ACADEMY. 



of about 25 cm. 



Tod 



r-id, 



Kct 



A 



The region under observation was limited by means of 

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

 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 

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

 sponding distance, and viewed through a circular 

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

 interposed between the flame and the slit and as 

 near the former as practicable. 



The acetylene flame was adopted as a comparison 



standard for the fol- 



D 



l 



c 



A 



1I> 



the less refrangible 



lowing reasons : — 

 1. It possesses 

 a continuous spec- 

 trum, brighter in 

 regions than that of 



^ 



Figure 9. 



any other controllable source of light. 



2. The radiating material is finely di- 

 vided carbon, presumably of a character 

 not unlike that of the surface of the 

 untreated rod. 



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

 fuel (C 2 rl 2 ) 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 

 constaut in its intensity and color than any other fkime 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 the temperature of the acetylene flame, varying and in- 

 compatible statements are in existence. The temperature of combustion 



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

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

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



