On the Reciprocal Action of Diamagnetic Particles. 67 



magnet thus held experiences. Therefore I say it is in this case 

 repulsion. But it will be as much smaller in proportion to the 

 force experienced by the steel magnet, as it would be if an iron 

 wire were substituted for the bismuth core. Yet in this case the 

 repulsion on the bismuth is very slight, barely sensible, or per- 

 haps not at all sensible when the needle exhibits most energetic 

 signs of the forces it experiences. You know youi'self, by your 

 own experiments^ how very small is even the directive agency 

 experienced by a steel magnet placed across the lines of force 

 due to the bismuth core. You may judge how much less sensible 

 would be the attraction or repulsion it would experience as a 

 whole, if held along the lines of force ; and then think if the 

 corresponding force experienced by a fragment of bismuth sub- 

 stituted for it is likely to be verified by direct experiment or ob- 

 servation. I think you will admit that it is "incapable of veri- 

 lication," as well as " incontrovertible " by any collation of the 

 results of experiments hitherto made on diamagnetics. As to 

 the concluding paragraph of my letter which you quote, you do 

 me justice when you say you accept it as an expression of my 

 "personal conviction that the action referred to is too feeble 

 to be rendered sensible by experiment." I will not maintain its 

 unqualified application to all that can possibly be done in future 

 in the way of experimental research to test the mutual action of 

 diamagnetics under magnetic influence. On the contrary, I 

 admit that no real physical agency can be rightly said to be 

 " incapable of verification by experiment or observation;" and 

 I will ask you to limit that expression to experiments and obser- 

 vations hitherto made, and to substitute for the concluding para- 

 graph of my letter the following statement, written for publication 

 three days later, and published in the same Number of the Ma- 

 gazine as that to which you communicated my letter (Phil. Mag. 

 April 1855, p. 24^7). "The mutual influence" between rows of 

 balls or cubes of bismuth in a magnetic field, " and its effects " 

 in giving a tendency to a bar of the substance to assume a posi- 

 tion along the lines of force, " are so excessively minute, that they 

 cannot possibly have been sensibly concerned in any phsenomena 

 that have yet been observed ; and it is probable that they may 

 always remain insensible, even to experiments especially directed 

 to test them." 



I remain, my dear Sir, 



Yours very truly, 

 Dr. Tyndall. William Thomson. 



F2 



