178 Messrs. J. Tyndall and H. Knoblauch on the Deportment 



of uda; + vdy—d<p the movement is always rectilinear. The 

 equation (ll.)? even were it true, proves nothing; it remains 

 to demonstrate that, in the case above-mentioned, the lines of 

 displacement are always circular. 

 Florence, January 8, 1850. 



XXIV. On the Deportment of Crystalline Bodies bet'ween the 

 poles of a Magnet. By John Tyndall and Hermann 

 Knoblauch*. 



THE results obtained by Professor Pliicker of Bonn, in 

 his investigations upon crystals, induced us early in the 

 month of November last to commence a series of researches 

 in connexion with this subject. Our inquiries, so far as they 

 at present reach, form the subject of this paper. 



After a long series of trials, not necessary to be recapitu- 

 lated here, we arrived at the persuasion that no safe inference 

 could be drawn from experiments made with full crystals. It; 

 appeared necessary to examine the forces attributed to cry- 

 stalline, bodies in detail, one at a time, removing as far as pos- 

 sible all influences likely to interfere with the simple action of 

 this one. 



To attain this object we experimented with cubes : we had 

 one cut from tourmaline, in such a manner that the optical 

 axis of the crystal ran parallel to four sides of the cube ; on 

 suspending it between the poles and closing the circuit, the 

 optical axis set itself strongly equatorial ; thus corroborating 

 the law of Pliicker, which affirms that the optical axes of ne- 

 gative crystals are repelled. When, however, the same cube 

 was hung with the optical axis vertical, the influence of that 

 axis being thus destroyed, a preference was shown to one of 

 the diagonals of the horizontal face of the cube, not to be ex- 

 plained by the law mentioned ; this preference was more stri- 

 kingly exhibited in the case of the following two crystals. 



A cube of beryl, cut similarly to the tourmaline, being hung 

 with its optical axis vertical, one diagonal of its horizontal face 

 set itself axial ; only one diagonal could maintain this position, 

 the other was repelled. In dichroite this phaenomenon was 

 very strikingly exhibited; when hung with the middle line of 

 the optical axes vertical, one diagonal assumed the axial posi- 

 tion ; if, however, the circuit was closed when the other dia- 

 gonal chanced to lie from pole to pole, the latter seemed to 

 experience a repulsive shock, sufficient to make the cube spin 

 several times round upon its axis. 



* Coraraunicated by the Authors. 



