It will thus be seen that the system and the class are precisely those of the two active tartaric acids, which renders the case the more interesting. The usual appearance of the crystals is shown in Fig. 65, and the stereographic projection is given in Fig. 66, which will elucidate the symmetry more clearly, the plane of projection being the plane of symmetry The latter, however, in this class is inoperative, the two ends of the digonal symmetry axis, which runs perpendicularly to the plane of the paper, being differently terminated, as in the tartaric acids. The faces of the forms _o_ = {111} and _m_ = {011} were never found developed on the left side of the symmetry plane, that is, on the left side of the crystal as drawn in Fig. 65, the symmetry plane running perpendicularly to the paper vertically from front to back; they were only present on the right. Conversely, the faces of _s_ = {̄1̄11} and _n_ = {̄2̄11} were never found developed on the right, but only on the left of the plane of possible symmetry. Moreover, it was frequently observed that the right-hand faces (110) and (̄110) of the primary prismform _p_ were much more brilliant and truly plane than those on the left hand, (1̄10) and (̄1̄10), which were usually dull and often curved, as were also frequently the faces of the left-hand forms _s_ and _n_. The right-hand distinguishing forms _m_ and _o_, on the contrary, were generally most brilliant and gave admirable reflections of the goniometer signal-slit. [Illustration: FIG. 65.—Crystal of Calcium Dextro-Glycerate. ] [Illustration: FIG. 66.—Stereographic Projection of Calcium Dextro-Glycerate. ] The following table represents the results of the angular measurements, twelve different well-formed individual crystals having been employed. The angles marked with an asterisk were the important angles the mean observed values of which were accepted as correct, being the best measured angles, and which were therefore used as the basis of the calculations. _Table of Interfacial Angles of Calcium Glycerate._ Angle measured. No. of Limits. Mean Calculated. measurements. observed. {_ap_ = 100 : 110 42 52° 32′ − 54° 16′ 53° 29′ * {_pp_ = 110 : ̄110 20 72 7 − 73 33 73 4 73° 2′ {_ac_ = 100 : 001 13 68 22 − 69 42 69 3 69 6 {_cr′_ = 001 : ̄201 13 52 4 − 52 31 52 13 * {_r′a_ = ̄201 : ̄100 13 58 35 − 58 46 58 41 * _cm_ = 001 : 011 10 31 47 − 32 19 32 3 32 2 _r′n_ = ̄201 : ̄2̄11 2 29 43 − 29 48 29 45 29 47