II. GENERAL ASPECTS OF OPTICAL ACTIVITY. A. Classification of Active Substances. § 7. Circular polarizing substances may be divided into two classes : (1) Bodies which only in a crystalline state possess the property of rotating the plane of polarization, and which lose this property entirely when brought (either by solution or fusion) into an amorphous condition. Up to the present, only a few such active crystalline substances are known. In the table annexed these substances are recorded, along with the angle a, through which the ray D, or mean yellow light j (jaune), is rotated on passing through a plate of each substance 1 millm. in thickness. The crystals of these substances are, without exception, either single-refracting (regular) or uniaxial double-refracting (hexagonal or quadratic). In the latter, the optical power is only displayed in the direction of the principal axis, and we have therefore to use plates cut perpendicularly thereto. Moreover, every one of the above specified substances occurs both in right-rotating and leftrotating crystals, and the amounts of deviation are exactly the same for plates of equal thickness. Several of these substances-quartz, sodium periodate, dithionates, guanidine carbonate, and matico-camphor, give external evidence of the possession of this property by the existence of hemihedral or tetartohedral faces, which are right-handed or lefthanded according as the crystal rotates to the right or left. § 8. (2) Bodies which display rotatory power when dissolved, and, consequently, in the amorphous state. The substances of this class are, without exception, carbon-compounds, and either occur naturally in vegetable or animal organisms, or as derivatives from these by simple metamorphoses. No inorganic substance is known which in solution exhibits rotatory power, and it would seem that this property is a peculiar attribute of the carbon-atom. The substances in this class are either right-rotating (+) only or left-rotating (—) only, with the exception of a few which manifest the power in both directions. The subjoined table contains as complete a list as possible of all active substances known up to the present time, with their most important derivatives, and also, in the last column, a list of compounds which, although nearly related to these substances, are inactive : |