by the employment of different Resistances. 527 



breaking the circuit, which circulate alternately to and fro. 

 Since then, Gassiot* has called attention to the fact that we 

 obtain a luminous appearance in tubes prepared by his method, 

 on establishing the primary current, provided we use ten or 

 more elements for the production of this current. 



It was probable, therefore, that, if a test-tube were inserted 

 which contained only a short stratum of very rarefied air, on esta- 

 blishing the principal current, generated in this case by two of 

 Bunsen's elements, an induced current would be formed. This 

 was found to be the case ; for if the current was established by 

 dipping the platinum wire of the contact-breaker only once with 

 the hand into the amalgam, a luminous appearance was obtained 

 in the test-tube, which, however, was considerably weaker than 

 that produced by breaking the circuit. The opposite currents, 

 observed with a very trifling resistance, depend therefore partly on 

 the induced current which originates on completing the circuit. 

 Still, I believe them to depend only partly thereon ; for the cur- 

 rent generated by once completing the circuit and then breaking- 

 it, also produces negative light at both polar wires. It might 

 certainly be affirmed that we could produce no single breaking 

 of the contact, that there is closing and breaking in succession ; 

 still there remains the remarkable fact, that by a single break- 

 ing of the contact the luminous appearance in the test-tube was 

 always the same, whether the separation was produced quickly, 

 by withdrawing a platinum point from the amalgam, or slowly, 

 by disconnecting two copper surfaces like those in the rheotrope. 



The following observation likewise shows the probability of 

 the production of opposite currents in the induction- wire by a 

 single breaking of the circuit. 



It has just been mentioned that, if we choose such a separa- 

 tion of the polar wires in the air-tube that the employment of 

 the small induction-apparatus produces single currents, and 

 if we then replace this apparatus by the large coil, opposite 

 currents are obtained. If, whilst the small induction-coil is 

 in action, we first of all observe in the well-rarefied air-tube 

 the negative wire, it appears to be covered to a considerable 

 length with bluish light, whilst the positive, on the other hand, 

 is entirely non-luminous. If we then employ the large induc- 

 tion-apparatus, a much smaller part of the negative wire is 

 blue, whereas now a portion of the positive wire shines with this 

 colour ; the wires comport themselves just in this way if the 

 circuit be broken once. It can scarcely be assumed that by such 

 a single breaking of the contact a re-establishment of the circuit 

 takes place which generates as strong a current as that which 

 arises by the regular closing of the circuit. If, therefore, it is 

 * Phil. Mag. vol. xvi. p. 307. 



