aiid tJieir relation to Crystalline Form and to the Optic Axes. 449 



the means which an increased acquaintance with the subject 

 placed at my disposal. It was necessary that the facts which 

 led to my hypothesis should be multiplied, and by new optic and 

 magnetic experiments rendered irrefragable. 



5. Beforehand, however, it was necessary to ascertain with greater 

 accm-acy the nature of the force to which the phsenomena were 

 due. I have stated the question with perfect distinctness in the 

 programme, but had not at the time an answer to it. Whence 

 does it arise, that a prism of tourmaline suspended horizontally 

 between the pointed poles is attracted when the latter are near 

 each other, but recedes from them when the poles are removed 

 to a greater distance, the mass of the crystal being, however, 

 still attracted ? The centre of gravity of each half retires as far 

 as possible from the poles when the tourmaline is suspended 

 from its middle ; and when hung so as to be able to rotate round 

 one end, the centre of gravity of the entire mass recedes from 

 the nearer and more strongly attracting pole. 



6. We have therefore first of all an attraction of the mass of 

 the crystal and repulsion of a certain direction through the same. 

 This is the expression which describes the phaenomena as they 

 exhibit themselves. Quite identical therewith is the statement 

 that the ciystal is differently attracted in dififerent directions ; 

 as such an action can be mechanically resolved into a uniform 

 attraction of the entire mass, and an attraction or repulsion of a 

 certain direction. Not an iota more is explained by the latter 

 assumption as long as we do not enter upon the natui-e of the 

 active forces. 



IMessrs. Tyndall and Knoblauch, who have published their 

 investigations on the deportment of crystalline bodies both in 

 these Annals and in two memoirs in the Philosophical Magazine, 

 consider that this difference of attraction contradicts the expla- 

 nation which I have given. I, however, on my side, had before 

 expressed the same modified conception in the programme : — 



"Quo modo enim phenomena explicare vis ? Non displiceret 

 nobis ponere, in sethere gyros Ampericos inductione, una aut 

 altera directione, circum axes facilius et validiores nasci, aut 

 quod idem, crystallum inductione magneticum, polos secxmdum 

 axium opticorum directionem adipisci : nisi hsec nova videndi 

 ratio etiam attractioncm aut repulsionem massse posccret, solo 

 axium situ mutatam. Quse actio diversa non observatur." 



7. The last assertion refers to direct experiments which seemed 

 to speak against the assumption of an attraction which varied 

 with the direction. I quotes these experiments from the above 

 memoir of December 1849, in which they have been more lately 

 inserted : — 



" From a very beautiful crystal of sulphate of iron I cut a 



