182 Professor TyndaiVs further Researches 



magnetic impurity attaching itself to the paper which covered 

 them^ a number of fourpennv pieces were procured, washed in 

 ammonia and water, and enclosed in thin glass tubes. The fol- 

 lowing were the results :— 



XXX. 



Silver cylinders (fourpenny pieces). 

 Current direct. Current direct. Current direct. 



Position 1. 490 Position 2. 565 Position 3. 660 



Here also we have a very considerable action indicative of mag- 

 netic polarity. On examining the cylinders between the poles 

 of an electro-magnet, they were found decidedly magnetic. 

 This, therefore, appears to be the common character of our silver 

 coins. The tubes which contained the pieces were sensibly neutral. 

 Knowing the difficulty of demonstrating the existence of dia- 

 magnetic polarity in ordinary insulators, M. Matteucci suggested 

 that insulated fragments of bismuth ought to be employed, the 

 insulation being effected by a coat of lac or resin. I constructed 

 a pair of cylinders in accordance with the suggestion of M. Mat- 

 teucci, Tne following are the results they yielded with a direct 

 current :— 



XXXI. 



Position 1. 730 Position 2. 750 Position 3. 768 



Here we have a very marked action, but the polarity indicated 

 is magnetic polarity. On subsequent examination, the cylinders 

 proved to be magnetic. This was due to impurities attaching 

 themselves to the resin. 



But the resin may be done away with and the powdered metal 

 still rendered an insulator. This thought was suggested to me 

 by an experiment of Mr. Faraday, which I will here describe. 

 Referring to certain effects obtained in his investigations on sup- 

 posed diamagnetic polarity, he writes thus : — '' If the effect were 

 produced by induced currents in the mass, division of the mass 

 would stop these currents and so alter the effect j whereas, if 

 produced by a true diamagnetic polaritf/, division of the mass 

 would not affect the polarity seriously or in its essential nature. 

 Some copper filings were therefore digested for a few days in 

 dilute sulphuric acid to remove any adhering iron, then well- 

 washed and dried, and afterwards warmed and stirred in the air, 

 until it was seen by the orange colour that a very thin film 

 of oxide had formed upon them ; they were finally introduced 

 into a glass tube and employed as a core. It produced no effect 

 whatever, but was as inactive as bismuth." (Exper. Resear.2658.) 

 . Now when bismuth is powdered and exposed to the action of 



