and their relation to Crystalline Form and to the Optic Axes. 457 



and magnesia^ the axis of which is attracted, as in the case of the 

 impure spar examined first bj^ IMessrs. Tyndall and Knoblauch ; 

 oxide of iron, sti'ongly magnetic, axis attracted ; beryl, magnetic, 

 negative, axis repelled ; arseuiuret of lead, axis repelled ; diop- 

 tase, negative, axis repelled; bismuth, optic axis strongly 

 attracted; antimony, the optic axis stands as in bismuth, per- 

 pendicular to the plane of most eminent cleavage, and was for- 

 merly found to be repelled : the authors now suppose that the 

 repulsion was due to a slight admixture of iron ; arsenic, optic 

 axis attracted; the specimens formerly experimented with con- 

 tained iron ; ice, optic axis attracted, crystal positive ; quartz, 

 with a battery of ten of Grove's cells, showed no action ; the action 

 formerly obseiTcd was due to the contact of an iron hammer. 



The second class of crystals examined are those whose funda- 

 mental form is the rectangular prism. 



Sulphate of nickel was found to be negative, optic axis re- 

 pelled; molybdate of lead, vesuviau and arseniate of potash 

 also negative, optic axes repelled; zircon, positive, optic axis 

 attracted ; yellow ferrocyanide of potassium shows no action ; 

 acetate of copper and lime positive, optic axis attracted ; uranite, 

 negative, optic axis attracted, perhaps analogous to the case of 

 antimony ; one specimen of scapolite had its optic axis attracted, 

 a second specimen showed no action ; the crystal according to 

 Brewster is negative. 



Crystals possessing two optic axes were next examined, com- 

 mencing with those whose fundamental form is a right rhombic 

 prism. The deportment of these ciystals cannot be referred to 

 a single magnetic axis, as in the cases above described. If a 

 ciystal with one axis be suspended with the said axis vertical, 

 all directive action disappears : not so in the case of crystals with 

 two axes ; the plane of these axes sets itself always either axial or 

 equatoreal. The crystals were examined in the following order : 

 citric acid, negative, the magnetic axes, which coincide with 

 the optic axes, are repelled ; aiTagonite, a negative crystal, shows 

 the same deportment ; tartrate of potash and soda is positive, 

 and its magnetic axes are attracted ; anhydrite is in optical re- 

 spects positive, in magnetical negative ; in the case of topaz, with 

 a batteiy of ten cells, the authors were unable to detect any 

 action, the former results were due to a mixture of iron ; protosul- 

 phate of iron has one magnetic axis, which coincides with the axis 

 of maximum elasticity, and is attracted ; succinic acid has a single 

 magnetic axis, which is rej)ellcd; ferridcyanide of potassium 

 and diopside are both in optical respects positive, in magnetical 

 negative ; the plane of the axis sets equatoreal. 



Phil. Mag. S. 4. Vol. 1. No. 6. June 1851. 2 1 



