262 Professors Liveing and Dewar, On the circtimstances [Oct. 30, 



much less expanded and the emission much more nearly balanced 

 by the absorption. 



By inclosing the arc in a block of lime or magnesia we have 

 found its steadiness very greatly increased, and the mass of metallic 

 vapour which can be maintained at a temperature approaching to 

 that of the arc much enlarged, but it cannot be said that that tem- 

 perature is at all under control, and the walls of the crucible are 

 almost always cooler than the contents. 



V. We have lately used a carbon tube, passed quite through a 

 block of lime, and made it the positive electrode of the current, 

 while the other electrode is a carbon rod passed into the lime at 

 right angles to the tube so as to meet it in the middle. 



In this way the outside of the tube becomes intensely heated, 

 the heat is retained by the jacket of lime, and the interior of the 

 tube gradually rises in temperature, and attains in the central part 

 a very high point. By stopping the arc it can be made to pass 

 through the same stages of temperature in the inverse order. 

 Observations are made by looking down the perforation. When 

 the light issuing from the tube is projected by a lens on to the slit 

 of a spectroscope, the heated walls of the tube give at top and 

 bottom a continuous spectrum, against which various metallic lines 

 are seen reversed, while in the central part, when the tube is open 

 at the farther end, the spectrum is discontinuous, and the metallic 

 lines seen reversed against the walls at top and bottom appear as 

 bright lines. Plate V. is a negative photograph shewing this effect. 



The apparatus employed is thus constructed : A rod of carbon, 

 a in the figure, 15 millims. in diameter, perforated down its axis 



with a cylindrical hole 4 millims. in diameter, is passed through a 

 hole in a lime block d, and is connected by means of a copper clip 

 with the positive electrode of a Siemens dynamo-electric machine ; 



