The Molecular Tactics of a Crystal

THE MOLECULAR TACTICS OF A CRYSTAL
LORD KELVIN
London HENRY FROWDE Oxford University Press Warehouse Amen Corner, E.C.
New York MACMILLAN & CO., 66 FIFTH AVENUE
BY LORD KELVIN, P.R.S. PROFESSOR OF NATURAL PHILOSOPHY IN THE UNIVERSITY OF GLASGOW AND FELLOW OF PETERHOUSE, CAMBRIDGE Being the Second Robert Boyle Lecture, delivered before the Oxford University Junior Scientific Club on Tuesday, May 16, 1893 / WITH TWENTY ILLUSTRATIONS Oxford AT THE CLARENDON PRESS 1894
Oxford PRINTED AT THE CLARENDON PRESS BY HORACE HART, PRINTER TO THE UNIVERSITY
§ 2. Consider now any particular person C (Fig. 1) on any intermediate floor, D and D′ his nearest neighbours, E and E′ his next nearest neighbours all on his own floor. His next next nearest neighbours on that floor will be in the positions F and F′ in the diagram. Thus we see that each person C is surrounded by six persons, DD′ , EE′ and FF′ , being his nearest, his next nearest, and his next next nearest neighbours on his own floor. Excluding for simplicity the special cases of rectangular grouping, we see that the angles of the six equal and similar triangles CDE , CEF , &c., are all acute: and because the six triangles are equal and similar we see that the three pairs of mutually remote sides of the hexagon DEFD′E′F′ are equal and parallel.
Fig. 1
Fig. 2.
§ 5. By adding ball after ball to such a cluster of thirteen, and always taking care to place each additional ball in some position in which it is properly in line with others, so as to make the whole assemblage homogeneous, we can exercise ourselves in a very interesting manner in the building up of any possible form of crystal of the class called ‘cubic’ by some writers and ‘octahedral’ by others. You see before you several examples. I advise any of you who wish to study crystallography to contract with a wood-turner, or a maker of beads for furniture tassels or for rosaries, for a thousand wooden balls of about half an inch diameter each. Holes through them will do no harm and may even be useful; but make sure that the balls are as nearly equal to one another, and each as nearly spherical, as possible.