94 OUTLINES OF PHYSICAL CHEMISTRY
allowed to crystallise by spontaneous evaporation. The crystals of the doable sodium ammonium salt which separate out are of two sorts and are recognised by the hemihedral faces which they possess. The crystals are then mechanically separated, and it is found that one sort is dextro-rotatory and gives dextro-tartaric acid, whilst the other sort is l»vo-rotatory and gives the laBvo-acid.
In order that this decomposition of the double salt may take place, the crystallisation must go on at a temperature below 28°. At higher temperatures a double sodium ammonium racemate is formed.
Penicillium glaucum destroys by preference the dextro-acid ; this offers us another means of decomposing racemic acid.
8. The Icevo-rotatory acid.
We have just seen how it is prepared. It is very similar to the dextro-acid, but rotates the plane of polarisa- tion to the left. Its salts are similar to those of the dextro-acid, but are lffivo-rotatory.
Dextro- and laBvo-tartrates are frequently isomorphous, but show opposite hemihedral facets (mirror-images of each other).
4. The inactive acid.
To obtain this, the dextro-acid is heated with a tenth of its weight of water to 165° for two days. The mixture obtained consists of dextro-acid, racemic acid, and the inactive acid. The isolation of this last is based on the solubility of its acid potassium salt (the other acid tartrates are only sparingly soluble in water).
Substances with four asymmetric carbon atoms. — For hexose
ch 2 (oh).Ch(oh).Ch(oh).Ch(oh).Ch(oh).coh.
sixteen (2 4 ) active modifications are possible as well as eight inactive racemic forms.
In the case of the hexites the similarity of the end-
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