of the Ilame and Furnace Spectra of Iron. 213 



space between the bars surrounding the exposed portion 

 of the carbon tube is filled up with carborundum powder. 

 An observation tube of carbon, about 2 inches long and 

 having a bore of |- inch, is fitted into the furnace-tube end 

 facing the projection-lens. The object of this tube is to 

 prevent the brilliant light, radiated by the walls of the furnace, 

 from reaching the lens. The other end of the furnace-tube is 

 likewise provided with a carbon tube of § inch bore and 

 about 3 to 4 inches long. Into the free end of this tube is 

 fitted a brass one, through which gases can be passed into 

 the furnace. At first, furnace-tubes 12 inches in length 

 were employed; but, after some trials, it was found that 

 for the purpose of the present research tubes only 6 inches 

 long did equally well. Thus with these shorter tubes the 

 heated length of furnace was only 4 inches. The furnace 

 was always set up in such a way that the axis of the tube was 

 in a line with the optic axis of the spectrograph collimator, 

 the middle part of the furnace and the spectrograph slit being 

 at the foci of the projection objective. The spectrograph was 

 the same one as that used m my work on flame spectra*. All 

 experiments were conducted at atmospheric pressure. The 

 furnace was heated by means of continuous current ranging 

 from 160 to 600 amperes. The potential difference between 

 the ends of the effective portion of the 6-inch tube varied 

 from 6 to 13 volts. The temperatures were measured by 

 means of a Wanner pyrometer, which could be directed upon 

 the middle part of the interior furnace- wall after withdrawal 

 of the carbon inlet tube from the end of the furnace. The 

 range of temperatures employed was comprised between 

 1500° and 2700° 0. Measurements were made in each 

 case, and, during long exposures, readings were taken at 

 regular intervals of time and, whenever necessary, the 

 current was readjusted in order to keep the temperature 

 constant. The pieces of metal to be vaporized were placed 

 along the bottom of the furnace-tube. In the case of iron 

 the arc method, former^ used for feeding flames, was suc- 

 cessfully applied to the furnace. As will be remembered^ 

 this method consists in passing a steady current of air or 

 oxygen through a glass bulb, which encloses an arc burning 

 between iron poles. The issuing air, which carries in sus- 

 pension the finely divided material from the arc (oxide of 

 iron), is slowly passed through the furnace-tube. 



* Hemsalech, I, c. I. p. 7. 



