380 EXPERIMENTAL INVESTIGATIONS. 



phorus succeeds best ; and with this is seen a remarkable 

 effect on the powder or smoke of allotropic phosphorus 

 (which is always formed when the striae are observed) : this 

 smoke traverses from pole to pole from the negative to the 

 positive side, showing, unless there be some latent optical 

 deception, a mechanical effect of the discharge under the cir- 

 cumstances.' 



Dr. Robinson, who has a little misunderstood my meaning 

 in this passage, says (' Proc. Royal Irish Acad./ December 

 1856) : ' Mr. Grove appears to think it arises from some vibra- 

 tion in the metal of a contact-breaker, which produces a fluc- 

 tuation in the inducing current.' I did not mean to say that 

 the effect was due to any peculiar vibration of the metal of the 

 contact-breaker, but to the interrupted and successive dis- 

 charges of the apparatus itself that the changes in the 

 character of the discharge attendant upon changes in the 

 action of the contact-breaker, as well as the frequent absence 

 of striae when only a single disruption was effected, afforded 

 evidence that the striae were connected with the multi- 

 plied discharges. My notice was, however, short and some- 

 what obscure, as, although I entered fully on the mattei 

 in the Section-room, I had prepared no memoir on the subject. 

 I still retain the opinion I then expressed, though I do not 

 assert it as a positive conviction ; the difficulty of proving it 

 arose from the circumstance that it was next to impossible to 

 produce, by the ordinary modes, a single discharge from the 

 induced coil, for the following reasons. When one extremity 

 of the wire of the primary coil is separated from the other, an 

 induced current is produced at the moment of disruption in the 

 secondary wire and a consequent discharge in the vacuum. But 

 at this same moment of disruption, the extra or induced current 

 in the primary wire itself finds a passage in the form of a spark 

 immediately the contact is broken, and this extra current 

 occasions necessarily a second induced current in the secondary 

 wire, which, having a ready path opened to it by the discharge 

 of the previous current, would be discharged through that 

 path, though it might not have tension enough to overcome 

 any great resistance. Although, therefore, these discharges 

 cannot be separated to the eye, it by no means follows that 



