and their relation to Crystalline Form and to the Optic Axes. 451 



In both the stream was produced by four cells of Grove's con- 

 struction. 



" The reflection that the polarity excited while the cube rested 

 on both the half-anchors must be of a very complicated nature, 

 induced me^ notwithstanding the great diminution of attraction, 

 to set the cube upon one pole only at that point where the force 

 is strongest. The attraction here amounted to 



0-187 grm. ; 

 it mattered not on which side the cube was placed, a diflference 

 was not perceptible, and certainly did not amount to 



0-01 grm. 

 " If a difference in the attraction of the crystal dependent on 

 the direction in which it approaches a pole of the magnet exist 

 — a question which I will not here decide — it is at all events so 

 small that the strong action exhibited by sulphate of iron cannot 

 be referred to it." 



8. I shall return to the consideration of these results, which 

 certainly stood in the way of the development of my theoretic 

 views, and merely di-aw attention to the fact, that we derive little 

 assistance from the assumption of an attraction or repulsion de- 

 pendent upon the direction, which acts at a distance from the 

 poles, but disappears in their immediate vicinity. 



9. Before I proceed further, I must caution against a false 

 interpretation of my ear Her views on this subject, the fault of 

 which, however, does not rest with me. When I spoke of the 

 attraction or repulsion of the optic axis, I had as little notion of 

 ascribing a mechanical action to an ideal direction, or of assuming 

 a mechanical action which was exerted on such an ideal direction, 

 as for instance M. Biot, when he says a ray is attracted by the 

 axis of rock-crystal and repelled by the axis of calcareous spar. 

 The expression I have used is rationally capable of this interpre- 

 tation only, that the mechanical action on the ci-ystal furnishes a 

 resultant which coincides with the direction of the optic axis ; 

 and if a doubt should still remain on this subject, a glance over 

 the last quotation from the programme wiU completely disperse it. 



10. With regard to the connexion between the common mag- 

 netic and diamagnetic forces with those which act upon the axes 

 of the crystals, I have not as yet uttered an opinion. I was 

 unable at the time to decide the mechanical question here im- 

 pending. When I used the expression, the action on the optic 

 axis of the ciystal decreases more slowly than the common action 

 on the mass, I meant thereby to express a fact merely. The 

 former could thus be only a modification of the latter, which 

 exhibited itself at greater distances. Quite otherwise was it with 

 regard to the coal and the mixed bodies, which exhibited appa- 



