Note 1130 — Chapter VII · (continued 2)

The difficulties introduced into the study of minerals by the discovery of isomorphism and polymorphism, are no doubt considerable; but M. Beudant (Minéralogie, Paris, 1841, p. 37) seems to me to exaggerate their effect upon ‘l'importance des formes crystallines.’ They are much more damaging to the purely chemical arrangement, because our implements for measuring the minute angles of crystals are still very imperfect, and the goniometer may fail in detecting differences which really exist; and, therefore, many alleged cases of isomorphism are probably not so in reality. Wollaston's reflecting goniometer has been long considered the best instrument possessed by crystallographers; but I learn from Liebig and Kopp's Reports, vol. i. pp. 19, 20, that Frankenheim has recently invented one for measuring the angles of ‘microscopic crystals.’ On the amount of error in the measurement of angles, see Phillips's Mineralogy, 1837, p. viii.