This subject, the main results and principles of which have now been elucidated, may well be closed with a reference to an interesting case of enantiomorphism and optical activity which the author has himself investigated,[15] and which is very similar to the case of the tartaric acids. It had been previously shown[16] by P. F. Frankland and W. Frew, that when calcium glycerate was submitted to the fermenting action of the _Bacillus ethaceticus_ one-half only of the glyceric acid was destroyed, and that the remaining half was optically active, rotating the plane of polarisation to the right. Now glyceric acid, CH_{2}.OH | CH.OH, | COOH has manifestly one so-called asymmetric carbon atom (that is, a carbon atom the four valencies of which are satisfied by attachment to four different monad elements or groups), that belonging to the CHOH group. There are consequently two possible arrangements of the molecule in space, probably corresponding to the two optically active varieties, namely, those represented, as far as is possible in one plane, as below, the asymmetric carbon atom (not shown in the graphic representation) being supposed to be at the centre of the tetrahedron, which is usually taken to represent a carbon atom with its four valencies. [Illustration] Dextro-glyceric acid itself proved to be an uncrystallisable syrup, but the calcium salt, Ca(C_{3}H_{5}O_{4})_{2}.2H_{2}O, was obtained in crystals sufficiently well-formed to permit of a complete crystallographic investigation, which the author undertook by friendly arrangement with Prof. Frankland. Although the acid itself is dextro-rotatory, aqueous solutions of the calcium salt are lævo-rotatory to the extent of –12.09 units of “specific rotation” for sodium light. The crystals were colourless well-formed prisms which proved to be of monoclinic symmetry, the best individuals being formed by very slow evaporation of the aqueous solution. They were terminated at both ends by pyramid and dome faces, and sometimes grew to the length of a centimetre. The actual crystal elements found after a full series of measurements were as under:— CALCIUM DEXTRO-GLYCERATE. _Crystal system_: monoclinic. _Class of Monoclinic System_: sphenoidal or monoclinic-hemimorphic. _Habit_: prismatic. _Monoclinic axial angle_: β=69° 6′. _Ratio of axes_: _a_ : _b_ : _c_ = 1.4469 : 1 : 0.6694. _Forms observed_: _a_ = {100}, _c_ = {001}, _r′_ = {̄201}, _p_ = {110}, _m_ = {011}, _o_ = {111} _s_ = {̄1̄11}, _n_ = {̄2̄11}.