24 Messrs. J. Tyndall and H. Knoblauch on the 



the equatorial position with great energy. The deportment 

 of the first model is that of scapolite ; of the second, that of 

 beryl. By using a thin layer of bismuth paste instead of the 

 magnetic sand, the deportment of saltpetre and topaz will be 

 accurately imitated. 



Our fundamental idea is, that crystals of one cleavage are 

 made up of plates indefinitely thin, separated by spaces indefi- 

 nitely narrow. If, however, we suppose two cleavages existing 

 at right angles to each other, then we must relinquish the notion 

 of plates and substitute that of little parallel bars; for the 

 plates are divided into such by the second cleavage. If we 

 further suppose these bars to be intersected by a cleavage at 

 right angles to their length, then the component crystals will 

 be little cubes, as in the case of rocksaltand others. By thus 

 increasing the cleavages, the original plates maybe subdivided 

 indefinitely, the shape of the little component crystal bearing 

 special relation to the position of the planes*. It is an infer- 

 ence which follows immediately from our way of viewing the 

 subject, that if the crystal have several planes of cleavage, but 

 all parallel to the same straight line, this line, in the case of 

 magnetic crystals, will stand axial ; in the case of diamagnetic, 

 equatorial. It also follows, that in the so-called regular cry- 

 stals, in rock-salt, for instance, the cleavages annul each other, 

 and consequently no directive power will be exhibited, which 

 is actually the case. Everything which tends to destroy the 

 cleavages tends also to destroy the directive power; and here 

 the temperature experiments of Mr. Faraday receive at once 

 their solution. Crystals of bismuth and antimony lose their 

 directive power just as they melt, for at this particular instant 

 the cleavages disappear. Iceland spar and tourmaline, on the 

 contrary, retain their directive power, for in their case the 

 cleavages are unaffected. The deportment of rock crystal, 

 whose weakness of action appears to have taken both Mr. 

 Faraday and M. Pliicker by surprise — as here the optic axis 

 force, without assigning any reason, has thought proper to 

 absent itself almost totally — follows at once from the homo- 

 geneous nature of its mass; it is almost like glass, which pos- 

 sesses no directive power; its cleavages are merely traces of 

 cleavage. If, instead of possessing planes of cleavage, a cry- 

 stal be composed of a bundle of fibres, the forces may be ex- 

 pected to act with greater energy along the fibre than across 

 it. Anything, in short, that affects the mechanical arrange- 

 ment of the particles will affect, in a corresponding degree, 

 the line of elective polarity. There are crystals which are 

 both fibrous and have planes of cleavage, the latter often per- 

 * See the last note at bottom of page 23. 



