An introduction to chemical crystallography / by P. Groth, Authorised translation by Hugh Marshall.

  • Groth, P. (Paul), 1843-1927.
Date:
1906
Catalogue details

Licence: Public Domain Mark

Credit: An introduction to chemical crystallography / by P. Groth, Authorised translation by Hugh Marshall. Source: Wellcome Collection.

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    mind (loc. citi), but did not pursue it further) ; for atoms, like molecules, are possessed of specialised directions, as shown by their valency, by stereo-isomerism and other chemical relations, apart altogether from their possible compound nature and the localised electric charges (or the electric currents circulating about them in a definite manner) in which probably are to be sought the origin of those forces whose equilibrium determines the crystal structure. On that supposition all the atoms in any such partial system would be orientated parallel to one another only in the special case of their forming a simple space lattice ; in general the partial system will be formed by the inter¬ penetration of several space lattices which differ in the orientation of the atoms composing them. If, for example, three regular point systems of this kind, possessing similar coincidence movements and consisting one of sulphur atoms, another of twice as many potassium atoms, and the third of four times as many oxygen atoms, are fitted the one within the other in such a way that equilibrium is maintained, there results a crystal structure with the properties and chemical composition of crystallised potassium sulphate. This structure completely fulfils the condition of “regularity,” for within it there exist, at equal but immeasurably small distances, homologous posi¬ tions (those of the similarly orientated atoms of each of the space lattices), i.e.} “points, about each of which the mass distribution parallel to any chosen direction in the crystal is the same as about any other.” It is evident, however, that in such a structure “ mole¬ cules,” in the sense in which we speak of gaseous molecules, are altogether wanting, and it is a matter of choice what is to be called the unit of crystal structure or the crystal molecule; for, just as it is quite arbitrary which mass-points of a regular point system may be supposed to constitute a closer group, so there are various ways in which, with equal right, adjacent potassium, sulphur, and oxygen atoms may be supposed to constitute a K2S04