thick, one half of which (d in fig. 75) is made out of dextro-rotatory, the other half (g in fig. 75) of lævo-rotatory quartz.

The light then passes through the tube containing the solution in the position of the dotted line in fig. 74, then through a quartz plate cut perpendicularly to its axis (q in fig. 75), then through an arrangement called a compensator (r in fig. 75), then through a second nicol called the analyser, and lastly through a telescope (L in fig. 75).

The compensator consists of two quartz prisms (RR', fig. 75) cut perpendicularly to the axis, but of contrary rotation to the plate just in front of

D

FIG. 74. Soleil's saccharimeter.

them. These are wedge-shaped, and slide over each other, the sharp end of one being over the blunt end of the other. By a screw the wedges may be moved from each other, and this diminishes the thickness of quartz interposed; if moved towards each other the amount of quartz interposed is increased.

The effect of the quartz plate (d, g) next to the polariser (i in fig. 75) is to give the polarised light a violet tint when the two nicols are parallel to each other. But if the nicols are not parallel, or if the plane of the polarised light has been rotated by a solution in the tube, one half the field will change

FIG. 75.-Diagram of optical arrangements in Soleil's saccharimeter.

in colour to the red end, the other to the violet end of the spectrum, because the two halves of the quartz act in the opposite way.

The instrument is first adjusted with the compensator at zero, and the nicols parallel, so that the whole field is of one colour. The tube containing the solution is then interposed; and if the solution is optically inactive the field is still uniformly violet. But if the solution is dextro-rotatory the two halves will have different tints; a certain thickness of the compensating quartz plate which is lævo-rotatory must be interposed to make the tint of the two halves of the field equal again; the thickness so interposed can be read off in amounts corresponding to degrees of a circle by means of a vernier

and scale (E in fig. 75) worked by the screw which moves the compensator. If the solution is lævo-rotatory, the screw must be turned in the opposite direction.

Zeiss's polarimeter is in principle much the same as Soleil's; the chief difference is that the rotation produced by the solution is corrected, not by a quartz compensator, but by actually rotating the analyser in the same direction, the amount of rotation being directly read off in degrees of a circle.

Laurent's polarimeter is a more valuable instrument. Instead of using daylight, or the light of a lamp, monochromatic light (a sodium flame pro

duced by volatilising common salt in a colourless gas flame) is employed; the amount of rotation varies for different colours; and observations are recorded as having been taken with light corresponding to the D or sodium line of the spectrum. The essentials of the instrument are, as before, a polariser, a tube for the solution, and an analyser. The polarised light before passing into the solution traverses a quartz plate, which, however, covers only half the field, and retards the rays passing through it by half a wave-length. In the 0° position the two halves of the field appear equally

illuminated in any other position, or if rotation has been produced by the solution when the nicols have been set at zero, the two halves appear unequally illuminated. This is corrected by means of a rotation of the analyser that can be measured in degrees by a scale attached to it.

The specific rotatory power of any substance is the amount of rotation in degrees of a circle of the plane of polarised light produced by 1 gramme of the substance dissolved in 1 c.c. of liquid examined in a column 1 decimetre long.

= rotation observed.

=

=

=

weight in grammes of the substance per cubic centimetre.

length of tube in decimetres.

specific rotation for light with wave-length corresponding to the D line (sodium flame).

In this formula + indicates that the substance is dextro-rotatory, - that it is lævo-rotatory.

If, on the other hand, [a], is known, and we wish to find the value of w, then

This instrument is one in which a spectroscope and polarising apparatus are combined for the purpose of determining the concentration of substances

which rotate the plane of polarised light, It was invented by E. v. Fleischl for the estimation of sugar in diabetic urine. Its chief advantage is that no difficulty arises in forming a judgment as to the identity of two coloured

surfaces, as in Soleil's saccharimeter, or of two shades of the same colour, as in Laurent's instrument. The light enters at the right-hand end of the 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 lævo-rotatory, 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, ƒƒ, 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 ƒƒ being empty or filled with water or some other optically inert substance, two spectra are seen, one over the other, but each shows 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 ƒ 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 substance in f. 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 ƒ, 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 ƒ ƒ (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 dextro-rotatory, and the other tartaric acid differing from the common variety in being lævorotatory. 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 hand 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 dextro-rotatory power, while the others were lævo-rotatory. Pasteur further showed that if the mould Penicillium glaucum is grown in a solution of racemic acid, dextro-tartaric acid first disappears, and the lavo-acid alone remains. The subject remained in this condition for many years; it was, however, conjectured that probably there is some molecular condition corresponding to the naked-eye crystalline appearances which

produces the opposite optical effects of various substances. What this molecular structure is, was pointed out independently by two observers--Le Bel in Paris, and Van 't Hoff in Holland-who published their researches within a few days of each other. They pointed out that all optically active bodies contain one or more asymmetric carbon atoms, i.e. one or more atoms of carbon connected with four dissimilar groups of atoms, as in the following examples :

The question, however, remained--do all substances containing such atoms rotate the plane of polarised light? It was found that they do not; this is explained by Le Bel by supposing that these, like racemic acid, are compounds of two molecules-one dextro-, the other lævo-rotatory; that this

was the case was demonstrated by growing moulds, the fermenting action of which is to separate the two molecules in question. Then the other question -how is it that two isomerides, which in chemical characteristics, in graphic as well as empirical formulæ, are precisely alike, differ in optical properties? —is explained ingeniously by Van 't Hoff. He compares the carbon atom to a tetrahedron with its four dissimilar groups, A, B, C, D, at the four corners. The two tetrahedra represented in fig. 78 appear at first sight precisely alike ; but if one be superimposed on the other, C in one and D in the other could never be made to coincide. This difference cannot be represented in any other graphic manner, and probably represents the difference in the way the atoms are grouped in the molecule of right- and left-handed substances respectively.

MERCURIAL AIR-PUMPS

Pflüger's Pump.-l is a large glass bulb filled with mercury; from its lower end a straight glass tube, m, about 3 feet long, extends, which is connected by an india-rubber tube, n, with a reservoir of mercury, o, which can be raised or lowered as required, by a simple mechanical arrangement. From the upper end of the bulb, l, a vertical tube passes; above the stopcock, k, this has a horizontal branch, which can be closed by the stopcock, f. The vertical part is continued into the bent tube, which dips under mercury in the trough, h. A stopcock, j, is placed on the course of this tube. Beyond f the horizontal tube leads into a large double glass bulb, a b; a mercurial gauge, e, and a drying-tube, d, filled with pieces of pumice-stone moistened with sulphuric acid, are interposed. a is called the blood-bulb, and the