120 Profs. G. D. Liveing and J. Dewar. [June 15, 



block d, and is connected by means of a copper clip with the positive 

 electrode of a Siemens dynamo-electric machine ; another carbon rod 

 b, unperforated, is passed into the lime block through a second hole 



at right angles to the first, so that the end of the rod b meets the 

 rod a in the middle of the block of lime. The rod b is connected 

 with the negative electrode of the dynamo machine, and after contact 

 is made between the two carbons is raised a little so that the arc 

 discharge continues between the two carbon rods within the block of 

 lime or magnesia. In this way the outside of the rod or tube, a, 

 becomes intensely heated, the heat is retained by the jacket of lime, 

 and the interior of the tube gradually rises in the central part to a 

 very high temperature. 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. 



By passing a small rod of carbon c into the perforation from the 

 farther end, a luminous background can be obtained all across the 

 field, and then, as the walls of the tube are hotter than the metallic 

 vapours between them and the eye, the metallic lines are only seen 

 reversed. A very slight alteration in the position of the carbon rod 

 makes the lines disappear, or reappear, or show reversal, and as the 

 core is adjusted by eye observation before photographs are taken, all 

 the conditions of the experiments are thoroughly known and are 



