]60 TRANSACTIONS OV SECTION A. 



opportunity to adapt itself : and piezoelectric phenomena might have been 

 anticipated on the same lines. Thus, as there is not complete compensation 

 magnetically, an electrically neutralised crystalline body moving with high speed 

 of rotation through the jether would be expected to produce a magnetic field : 

 and a planet whose materials have crystallised out in some rough relation to 

 the direction of gravity, or of its rotation, would possess a magnetic field. But 

 relativity forbids that a crystalline body translated without rotation at astrono- 

 mical speeds should exhibit any magnetic field relative to the moving system. 



The very extraordinary feature of the earth's magnetic field is its great 

 and rapid changes, comparable with its whole amount. Yet the almost absolute 

 fixity of length of the astronomical day shows extreme stability of the earth 

 as regards its material structure. This consideration would seem to exclude entirely 

 theories of terrestrial magnetism of the type of (2) and (3). But the type (1), 

 which appears to be reasonable for the case of the sun, would account for 

 magnetic change, s^udden or gradual, on the earth merely by change of internal 

 conducting channels : though, on the other hand, it would require fluidity and 

 residual circulation in deepseated regions. In any case, in a celestial body 

 residual circulation would be extremely permanent, as the large size would make 

 effects of ordinary viscosity nearly negligible. 



During the meeting, Models of Crystals, devised by Miss Nina 

 HosALi, were shown, as to which the following statement was issued: — 



The object of these models is 



(1) To illustrate the forms possible to crystals: 



(2) To show as clearly as possible the different kinds of symmetry possessed 

 by these forms ; and 



(3) To show how the forms are referred t-o crystallographic axes. 



Each model illustrates one of the thirty-two classes of symmetry, and 

 represents several crystal forms correctly orientated with regard to the crystallo- 

 graphic axes, the latter being shown by black threads. A model consists in 

 the first place of a glass envelope whose shape is that of some simple crystal 

 form, and within this envelope two or three other forms are represented by 

 means of coloured silk threads stretched over frameworks of thin copper wire. 

 By this means it is easy to make the forms intersect if necessary, and they 

 are readily distinguished from one another by the use of differently coloured 

 threads. 



The symmetry elements of the class represented by any model are shown 

 as follows : 



(a) The traces of the Planes of Symmetnj on the glass envelope are shown 

 by steel wires. 



(6) Axes of Symmetry are shown bj' ivJiite threads. (If an axis of symmetry 

 and a crystallographic axis are coincident, the white and black threads repre- 

 senting them are twisted together.) The degree of symmetry possessed by an 

 axis is indicated by small numbers attached to the thread near its ends. 



(c) When simultaneous rotation about an axis and reflection across a 

 perpendicular plane occur to produce Alternating Symmetry, the traces of the 

 plane on the glass envelope are shown by red and vjJiite twisted threads, and 

 the axis is shown by a white thread, its degree of symmetiy being indicated 

 by small numbers fixed to it and printed in red. 



(d) When the symmetry elements are such that the forms are Centro- 

 Symmetrical (i.e. when the faces occur in parallel j)airs), a couple of white beads 

 are placed at the centre of the model. 



The set of twenty-four models here exhibited represents twenty-one out 

 of the thirty-two classes and over seventy different forms. In many cases 

 different varieties of the forms may be pjoduced by rotating or inverting the 

 models, or by reflecting them in a mirror, and, when these modifications are 

 taken account of, the number of forms shown is brought up to about 140. 



