■developed beiwee?i Metals and fused Salts. 4'3g> 



Hitherto I have only described the currents produced 

 when the interposed salt is in a state of perfect fusion, but 

 before the salt becomes actually fused, electrical currents are 

 developed, whose direction no longer follows the simple law, 

 that has been before enunciated, but varies in the most sin- 

 gular and perplexing manner. After a long and tedious in- 

 vestigation, 1 have been completely baffled in my attempts to 

 discover the essential conditions upon which the directions of 

 these currents depend, and I shall therefore describe at pre- 

 sent only one or two experiments which will show the com- 

 plicated nature of the inquiry, and may, perhaps, draw the 

 attention of others to this curious part of electrical science. 

 In the investigation of these currents a very sensible galvano- 

 meter must be employed. ^ 



A small piatina spoon was partly filled with fused carbo- 

 nate of soda, and the end of a thick wire of the same metal 

 was introduced into the fused salt, metallic contact being care- 

 fully avoided. When the salt had cooled, the wire and spoon 

 were connected with the galvanometer. On applying a very 

 gentle heat to the bottom of the spoon, by means of a small 

 spirit-flame placed at a considerable distance, a current was 

 obtained from the spoon to the wire, or from the hot metal 

 to the cold ; this current was very feeble and could rarely be 

 maintained beyond a few minutes. By increasing the tem- 

 perature of the lower part of the spoon till the salt in contact 

 with it entered into fusion, while the portion surrounding the 

 cold wire was still in a solid state, a powerful current was ob- 

 tained from the wire to the cup, or from the cold metal to the 

 hot. When the temperature of the cup was still further raised 

 so as to fuse the whole of the salt, the current was of course 

 again reversed, being from the hot metal to the cold. It was 

 interesting to observe the violent manner in which from this 

 cause the needle of the instrument started from one extremity 

 of its scale to the opposite, on the slightest movement of the 

 flame. 



To the class of partially fused salts belongs heated glass, 

 which accordingly presented similar changes in the direction 

 of the current. Thus when a piatina wire was covered with 

 a very thin coating of glass, and another wire at a higher tem- 

 perature brought into contact with the glass, the current was 

 from the cold metal through the glass to the hot. If a thicker 

 piece of glass was interposed, the first current was from the 

 hot wire to the cold, but on raising the temperature a current 

 was obtained in the opposite direction. M. Becquerel had 

 already observed by means of a sensible gold-leaf electroscope 

 that when piatina wires at unequal temperatures are separated 



