194 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



instrument, is polarised by the nicol's prism b, and then passes through two- 

 quartz plates, cc, placed horizontally over each other. One of these plates is 

 dextrO", the other laevorotatory, and they are of such a thickness (7*75 mm.) 

 that the green rays between the E and b lines of the spectrum are circularly 

 polarised through an angle of 90°, the one set passing off through the upper 

 quartz to the left, the other through the lower to the right. The light then 

 continues through a long tube, ff, which contains 15 c.c. of the solution under 

 examination. It then passes through an analysing nicol d, and finally 

 through a direct-vision spectroscope, e. On looking through the instrument, 

 the tube ff being empty or filled with water or some other optically inert 

 substance, two spectra are seen, one over the other, but each show's a dark 

 band between E and b owing to the extinction of these rays by the circular 

 polarisation, produced by the quartz. The analyser can be rotated: a 

 vernier, g, is attached to, and moves with it, round a circular disc (seen in 

 section at h) graduated in degrees. The two bands in the spectra coincide 

 when the zeros of vernier and scale correspond. If now the tube / is filled 

 with an optically active substance like sugar, the bands are shifted, one to the 

 right, the other to the left, according to the direction of rotation of the sub- 

 stance in /. The rotation is corrected by rotating the analyser into such a 

 position that the two bands exactly coincide once more as to vertical position. 

 The number of degrees through which it is thus necessary to move the 

 analyser measures the amount of rotation produced by the substance in /, 

 and is a measure of the concentration of the solution. The degrees marked 

 on the circular scale are not degrees of a circle, but an arbitrary degree of 

 such a length that each corresponds to 1 per cent, of sugar in the given 

 length of the column of fluid in^ (177*2 mm.). 



RELATION BETWEEN CIRCULAR POLARISATION AND CHEMICAL 

 CONSTITUTION 



The first work in this direction was performed by Pasteur, and it was his 

 publications on this subject that brought him into prominence. He found 

 that racemic acid, which is optically inactive, can be decomposed into two 

 isomerides, one of which is common tartaric acid which is dextrorotatory, and 

 the other tartaric acid differing from the common variety in being laevo- 

 rotatory. The salts of tartaric acid usually exhibit hemihedral faces, while 

 those of racemic acid are holohedral. Pasteur found that, although all the 

 tartrate crystals were hemihedral, the hemihedral faces were situated on 

 some crystals to the right, and on others to the left band of the observer, so 

 that one formed, as it were, the reflected image of the other. These crystals 

 were separated, purified by recrystallisation, and those which exhibited 

 dextro-hemihedry possessed dextrorotatory power, whilst the others were 

 laevorotatory. Pasteur further showed that if the mould Penicillium glaucum 

 is grown in a solution of racemic acid, dextro-tartaric acid first disappears, 

 and the Isevo-acid alone remains. The subject remained in this condition for 

 many years ; it was, however, conjectured that probablj"^ there is some mole- 

 cular condition corresponding to the naked-eye crystalline appearances which 

 produces the opposite optical effects of various substances. What this mole- 



