Apparatus Illustrating Crystal Forms. 127 



and non-conductors should, on this theory, have their names 

 interchanged. However, such a paradox only arises because 

 one does not specify the kind of conduction or non-conduction 

 considered. Undoubtedly metals are non-conductors of elec- 

 tric force, and just for this reason they compel it under certain 

 circumstances to remain concentrated instead of becoming 

 dissipated, and thus they become conductors of the apparent 

 source of these forces, electricity, to which the usual termi- 

 nology has reference. 

 Karlsruhe, March 1889. 



XVIII. An Apparatus Illustrating Crystal Forms. By 



R. J. Anderson, M.A., M.B., Professor of .Natural History 



in Queen's College, Galway*. 



[Plate II.] 

 fTlHE apparatus by which I propose to illustrate crystal 

 J- forms consist of frameworks and cords and weights. 



The first piece of apparatus is figured in Plate II. fig. 1, and 

 consists of a frame made of wood. This is divided into two 

 compartments. One of these has, above, a slit half an inch 

 wide that runs from end to end ; in this slit a slide moves to 

 and fro, and can be fixed by means of a binding-screw at any 

 place. 



A slide of a similar kind moves in a slit in the lower part of 

 the framework ; this can be fixed by binding-screws in any 

 position. Pulleys are fixed at the ends of this compartment. 



Slips of wood run from end to end at the sides and carry 

 riding-slides. These slides have ^binding-screws and pulleys 

 whose sheaves revolve on a vertical axis fitted to them. 



A figure is easily constructed by carrying cords over the 

 pulleys. Single cords only are shown in the figure. This is 

 for the sake of distinctness. 



Starting from a!, which marks a ring connected with the 

 weight p, a single cord runs through j3' (ring), vf (ring), 

 8' y through ring a', through ?/, to be fixed to a weight. 



A second cord starting from y f runs through </, e', if, 

 through ring f/, and then across to e' through this ring to 

 hook up a weight p' . A third cord is fixed to h' and runs 

 through e f , ft' ', */, through B / and ft' to loop up another weight. 

 The actual tension-weights are fixed to the small rings, which 

 act as pulleys. 



Each rhombus has in this way a cord to itself, and the size 

 of the rhombus depends on the weights attached. The smaller 

 the rhombus the more cord is to spare. 



The figure shown is the regular octahedron if the axes be 

 * Communicated by the Physical Society : read April 13, 1889. 



L2 



