Piezo-electric Property of Quartz. 333 



I can imagine is as follows :— For brevity I shall explain it 

 in relation to the chemical constitution which, according to 

 present doctrine, is one atom of silicon to two atoms of oxygen. 

 The chemical molecule may be merely Si0 2 for silica in 

 solution or it may consist of several compound molecules of 

 this type, grouped together : but it seems certain that, in 

 crystallized silica (in order that the crystal may have the 

 hexagonally eolotropic piezo-electric property which we know 

 it has) the crystalline molecule must consist of three Si0 2 

 molecules clustered together ; or must be some configuration 

 three atoms of silica and three double atoms of oxygen 

 combined. As a ready and simple way of attaining the 

 desired result, take a cluster of three atoms of silicon and 

 three double atoms of oxygen placed at equal distances of 60° 

 in alternate order, silicon and oxygen, on the circumference 

 of a circle. 



The diagram, fig. 1, shows a crystalline molecule of this 

 kind surrounded by six nearest neighbours in a plane per- 

 pendicular to the axis of a quartz crystal. Each silicon atom 

 is represented by -f (plus) and each oxygen double atom by 

 — (minus). The constituents of each cluster must be supposed 

 to be held together in stable equilibrium in virtue of their 

 chemical affinities. The different clusters, or crystalline 

 molecules, must be supposed to be relatively mobile before 

 taking positions in the formation of a crystal. But we must 

 suppose, or we may suppose, the mutual forces of attraction 

 (or chemical affinity,) between the silicon of one crystalline 

 molecule and the oxygen of a neighbouring crystalline 

 molecule, to be influential in determining the orientation of 

 each crystalline molecule, and in causing disturbance in the 

 relative positions of the atoms of each molecule, when the 

 crystal is strained by force applied from without. 



§ 5. Imagine now each double atom of oxygen to be a 

 small negatively electrified particle, and each atom of silicon 

 to be a particle electrified with an equal quantity of positive 

 electricity. Suppose now such pressures, positive and negative, 

 to be applied to the surface of a portion of crystal as shall 

 produce a simple elongation in the direction perpendicular 

 to one of the three sets of rows. This strain is indicated by 

 the arrow-heads in fig. 1, and is realized to an exaggerated 

 extent in fig. 2. 



This second diagram shows all the atoms and the centres of 

 all the crystalline molecules in the positions to which they are 

 brought by the strain. Both diagrams are drawn on the 

 supposition that the stiffness of the relative configuration of 

 atoms of each molecule is slight enough to allow the mutual 



