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
THE
MOLECULAR TACTICS OF
A CRYSTAL
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
ON THE
MOLECULAR TACTICS OF A CRYSTAL
By LORD KELVIN, P.R.S.
§ 1. My subject this evening is not the physicalproperties of crystals, not even their dynamics; it ismerely the geometry of the structure—the arrangementof the molecules in the constitution of a crystal.Every crystal is a homogeneous assemblage of smallbodies or molecules. The converse proposition isscarcely true, unless in a very extended sense of theterm crystal (§ 20 below). I can best explain a homogeneousassemblage of molecules by asking you tothink of a homogeneous assemblage of people. To behomogeneous every person of the assemblage mustbe equal and similar to every other: they must beseated in rows or standing in rows in a perfectly similarmanner. Each person, except those on the borders ofthe assemblage, must have a neighbour on one sideand an equi-distant neighbour on the other: a neighbouron the left front and an equi-distant neighbourbehind on the right, a neighbour on the right frontand an equi-distant neighbour behind on the left. Histwo neighbours in front and his two neighbours behindare members of two rows equal and similar to the rows6consisting of himself and his right-hand and left-handneighbours, and their neighbours’ neighbours indefinitelyto right and left. In particular cases the nearestof the front and rear neighbours may be right in frontand right in rear; but we must not confine our attentionto the rectangularly grouped assemblages thus constituted.Now let there be equal and similar assemblageson floors above and below that which we havebeen considering, and let there be any indefinitelygreat number of floors at equal distances from oneanother above and below. Think of any one personon any intermediate floor and of his nearest neighbourson the floors above and below. These three personsmust be exactly in one line; this, in virtue of thehomogeneousness of the assemblages on the threefloors, will secure that every person on the intermediatefloor is exactly in line with his nearest neighboursabove and below. The same condition of alignmentmust be fulfilled by every three consecutive floors, andwe thus have a homogeneous assemblage of people inthree dimensions of space. In particular cases everyperson’s nearest neighbour in the floor above may bevertically over him, but we must not confine ourattention to assemblages thus rectangularly groupedi