292 Mr. Gr. A. Hemsalech on the Origin of the 



natural to try and obtain the desired effect by the simul- 

 taneous application of both thermal and electric forces, thus 

 reproducing the particular conditions which are believed to 

 exist in a resistance-tube furnace. To this end a special type 

 of plate-furnace was built which permitted the establishment 

 within the metal vapour of a potential gradient of any 

 required strength. The principle of this furnace is illus- 

 trated in fig. 3. Two graphite plates AB and CD are placed 



Fig. 3. 



+ 



Principle of Two-plate Furnace. 



with their flat sides parallel to each other at distances varying 

 from 3 to 10 mm. or more. At one end, A and C, the plates 

 communicate by means of a graphite block of the requisite 

 thickness. At the other end the plates remain insulated 

 from each other, and the extremities B and D are connected 

 to the mains. Now it is evident that when an electric 

 current is sent through the plates under these conditions a 

 potential gradient will be established within the space between 

 the plates, owing to the resistance of the latter. This gradient 

 will have a maximum value between B and D and it will 

 vanish at E. The magnitude of the gradient at the extre- 

 mities B and D for a given value of the lieu ting current and 

 for graphite plates of given sectional area, will vary directly 

 as the lengths of the plates, and inversely as the distance 

 between them. If the current sent through the plates 

 be of such strength as to raise the temperature of the 

 plates sufficiently to ionize the space between them, an 

 ionization current having a maximum value near the free 

 ends B and D will pass across the space. Further, if the 

 temperature attained be high enough to cause a piece of 

 metal, placed on the lower plate near D, to boil, the whole or 

 part of the ionization current will be carried by the metal 

 vapour, provided that both the electric conductivity of the 

 latter and the strength of the potential gradient be of the 

 requisite magnitude. Hence if the so-called high-temperature 

 line emission of iron vapour be really caused by the passage 

 of an electric current through the vapour, we should with a 

 furnace of this type observe its spectrum. Now, this is 



