18. CRYSTAL ANALYSIS

by Sir William Bragg

Clearly, there is some inherent property of atoms and molecules which causes them to associate into very regular structures if the circumstances allow. When, for example, a solution of copper sulphate is evaporating and the molecules begin to sort themselves out of the liquid and to associate in groups, in other words to crystallise out, the preciseness and constancy of the form which the grouping assumes tell us clearly that the solidification is not a mere gathering of the molecules into a jumbled heap. The faces of any crystal of copper sulphate make the same angles with each other as the corresponding faces of any other crystal of copper sulphate. It is true that the circumstances of growth may affect the relative rates at which the faces grow, but this only means that more molecules have been added in certain directions than in others. We can draw from these facts no other deduction than this, that there is in the structure a unit of pattern containing copper, sulphur, and oxygen atoms in the same proportions as that in which they occur in the molecule. Further, each unit must lie up against each neighbouring unit in precise fashion; the relative orientation of the two and their relative distance apart being the same throughout the structure. The particular form of the crystal is a direct consequence of the particular structure of the unit. It is not possible, however, to deduce the latter from the former, or it would have been done long ago. But now that the X-rays have come to our help, we obtain the power of looking past the outer appearance into the internal arrangement. The immense variety in the forms of various crystals, and the ease with which by precise measurements they can be distinguished from one another, are evidence of equal variety in the arrangement of the atoms and molecules in the unit and of equally precise distinctions. The structure of the single unit determines the properties of the solid substance. In many investigations of the behaviour of liquids and gases, in which the component molecules are not tied tightly together, the molecular form is of little consequence and it is sufficient to assume that it is spherical. But we cannot make this rough approximation when the molecules are associated into a solid crystal: for we should then blot out all those details which give each crystal its special characteristics.

(from Science in Writing, compiled by T. R. Henn (Harrap, 1960))