Page:Outlines of Physical Chemistry - 1899.djvu/107

 LE BEL AND VAN'T HOFF'S HYPOTHESIS

��The surface 1 — 2 — 4 thus presents a right-handed screw to

Let us now consider the behaviour of the four faces of the two enantiomorphic tetrahedra (figs. 19, a, and 19, b).

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Face 3—2—1 turns to the left 4-2-1 „ „ right 4—3—1 „ „ left 4—3—2 „ „ right

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��Face 3 — 2 — 1 turns to the right 4—2—1 „ „ left 4—3-1 „ „ right 4—3—2 ,. „ left

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��In both figures there are two faces of right rotation and two of left rotation. But the faces which turn to the right in one turn to the left in the other. It is very improbable that in a solution the molecules are all directed in a certain sense, and, consequently, the observed rotation

��1 The explanation of the phenomena of circular polarisation is due to Fresnel, This scientist has shown that in an optically active substance these phenomena are caused by the fact that the speed of transmission of a circular polarised wave varies according to the direction of its gyratory motion. Thus, if a linear polarised ray enters an active substance under normal incidence, i.e. perpendicu- larly, it is split up into two circular polarised rays R and b', a right one and a left one, and each of these is propagated at its own speed. On passing out from the substance these two rays, having the same speed in air, recombine to a linear polarised ray, whose plane of vibration makes an angle with that of the incident ray.

This explanation applies not only to quartz but also to substances containing an asymmetric carbon atom. For J these the helical structure of the molecules appears to us to be the probable cause of the unequal speeds of transmission of the waves b and b'.

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