��OUTLINES OF PHYSICAL CHEMISTRY
��The group 4, being more voluminous than group 2, is probably situated further away, with respect to the centre of gravity, from the central carbon atom (although its ' point of attachment ' may be closer). In the same way
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��group 2 is further away than group 1. From this it follows that the surface of 1—2 — 4 is a right-handed helical surface (one which we descend in passing in the direction in which the hands of a watch move). In order to make this clear, suppose a triangle representing the
surface 1 — 2 — 4 cut out of paper, and having a glass rod passed through its centre (fig. 20). Imagine the centre of gravity of the central carbon atom as situated on the glass rod behind the paper. The corner 4 should then be bent towards the front, i.e. removed further from the carbon atom. Similarly corner 1 is to be bent towards the back, i.e. made to approach the carbon atom. Corner 2, unfolded, will then occupy an intermediate position.
A ray of light passing in the direction indicated by the arrow will meet a right-handed screw-surface, and the same happens to a ray coming in the opposite direction.
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