450 Prof. Pliicker on the Magnetic Relations of the 



has obtained by two courses of reasoning in the Phil. Mag., 

 vol. xxxiv. pp. 54-57 and 57-59. I cannot concede that these 

 results have any weight against my position, because the rea- 

 soning from which they are derived takes for granted the 

 question in dispute; but I may adduce them in favour of it so 

 far as they exhibit inconsistencies. The first argument, which 

 professes to be a general consideration of a solitary wave of 

 arbitrary condensation, conducts to the result that the sum of 

 the condensations is exactly equal to the sum of the rarefac- 

 tions. Now if the reasoning be restricted to the case in which 

 the sum of the condensations is equal to the sum of the rare- 

 factions by the original disturbance, it ceases to be general, 

 and the result is a mere truism without meaning ; and if it be 

 not so restricted, it is impossible that the result can be true. 

 Mr. Stokes's argument cannot escape from this dilemma. The 

 second argument, which applies to a case in which condensa- 

 tion prevails over rarefaction, is included in the general argu- 

 ment above mentioned, if that be of any value, and its leading 

 by a different process to a different result is only another phase 

 of contradiction. 



As I consider that this hydrodynamical question has now 

 been so fully discussed that it is not likely to receive any ad- 

 ditional elucidation, as far as I am concerned the discussion is 

 closed. 



Cambridge Observatory, 

 May 23, 1849. 



LXVII. On the Magnetic Relations of the Positive and Nega- 

 tive Optic Axes of Crystals. By Professor Plucker of 

 Bo7i?if in a letter to, and communicated by, Dr. Faraday. 



ALLOW me, Sir, to communicate to you several new facts 

 which, I hope, will spread some light over the action of 

 the magnet upon the optic and magnecrystallic axes. 



L The first and general law I deduced from my last expe- 

 riments is the following one : — " There will be either repulsion 

 or attraction of the optic axes by the poles of a magnet, ac- 

 cording to the crystalline structure of the crystal. If the 

 crystal is a negative one, there will be repulsion ; if it is a po- 

 sitive one, there will be attraction." 



The crystals most fitted to give the evidence of this law are 

 diopside (a positive crystal), cyanite, topaz (both negative), 

 and other ones, crystallizing in a similar way. In these cry- 

 stals the line (A) bisecting the acute angles made by the two 

 optic axes, is neither perpendicular nor parallel to the axis (B) 

 of the prism. Such a crystal, suspended horizontally like a 



