Non-conducting Bodies in Electric Induction. 17 



be doubted that the superior face was negatively excited, the 

 inferior face was found also negative. On the contrary, had the 

 shell-lac disc lain upon an uninsulated metallic disc during the 

 rubbing, and after that the negative electricity of the supe- 

 rior face was destroyed by the application of a flame (or the 

 touching with a metallic plate, as instantly will be seen), the 

 inferior face was found to be positive. After destroying this 

 positive electricity, the superior face was again negative, and 

 thus continuing, alternately one face could be made positive, the 

 other negative. This experiment gave me the means to obtain 

 easily an electrophorus with a positively excited cake. For that 

 purpose the cake was laid on its uninsulated metaUic mould, 

 strongly rubbed with fur and inverted in the mould, so that the 

 not-rubbed face was uppermost. When this cake was covered 

 with its covercle (a metallic disc), I had an electrophorus which 

 gave negative electricity, instead of the common electrophorus 

 giving positive electricity. 



With respect to your fourth and last remark, I concede entirely 

 that it is not correct to consider a small portion of a large piece 

 of shell-lac as equal to a small piece of the same, which I have 

 done at p. 405 of my paper. But I believe to have rendered 

 this incorrectness innoxious by referring to the end of my paper, 

 where I have explained why a partial introduction of the non- 

 conducting plate between the inductric and the inducteous body 

 apparently diminishes the induction and strengthens it by com- 

 plete interposition. I still consider this opposite effect of one 

 and the same plate, together with the fact that the placing of the 

 plate at the side of the inductric body increases the induction 

 (p. 411 at the bottom), very difficult to be explained by your 

 theory of induction. 



I have little hope to persuade you, my dear Sir, to modify 

 your views on the action of insulators in electric induction, and, 

 I confess, if I could I would scarcely wish it. The great philo- 

 sopher works best with the help of his own conceptions, his self- 

 made tools, whose imperfections he avoids by dexterous applica- 

 tion. But these tools, so efficacious in his hand, are not only 

 useless but very dangerous in the hands of others, and you know 

 what mischief, for instance, the conceit of electric screening has 

 lately done in the hand of the since deceased Italian philosopher. 

 You will therefore not blame me if I follow the publication of 

 your remarks by my reply. I cannot have any objection against 

 the mode and place which you choose for this publication, and I 

 know that immediately after the appearance of your letter Prof. 

 Poggendorff will give a translation of it in his ' Annals.' 

 • I am, ray dearest Sir, 



Yours most faithfully, 



Phil! Mag. S. 4. Vol. 11. No. 69. Jan. 1856. ' C 



