The deposit found in wine-casks, named tartar or argol, is the potassium salt of four acids, to which the generic name tartaric has been given. But it has long been known that although all these acids are represented by the formula C₄H₆O₆, they differed in physical properties. In order to understand the nature of this difference, a short explanation must be given of the nature of polarised light.

A mineral named tourmaline is known, which is sometimes used as a gem. Its colour is usually green, and it occurs in fairly large transparent crystals. If a slice of this mineral be cut, it is to all appearance quite transparent, allowing light to pass with only a slight diminution in its intensity, as would be the case if the light were to pass through a plate of somewhat dull greenish glass. Through two parallel plates of tourmaline, if held in a certain position, light still passes with scarcely diminished intensity ; but if the one plate be slid round on its neighbour, so that it has performed a partial revolution of 90°, the two plates in this position are no longer transparent, but cut off light and become practically opaque. The light, after passing through the first plate, is said to be polarised; and it is possible to extinguish this polarised light by interposing a second plate placed in a certain position.

It is now certain that the phenomena of light are to be attributed to the propagation of waves in a fluid which penetrates all space, even the interior of solids; a fluid without weight, possibly composed of particles, which, however, must not be confused with the atoms or molecules of ponderable bodies. This fluid is termed ether. Now waves may be caused in ether by other agencies than light. When an ordinary Leyden jar is discharged, the process of discharging it is not instantaneous ; the spark which passes between the knob of the electrical " tongs " used to discharge it and the knob of the jar is merely one of a number which pass to and fro between the knob of the tongs and the knob of the jar until electrical equilibrium is established. The passage of such sparks causes waves to be propagated through the ether, waves which differ from those of light only in their much greater length. Such waves, whether of light or of electric disturbance, are propagated at right angles to the direction of their oscillation ; they resemble waves cn the sea in this respect, but differ inasmuch as sea-waves oscillate merely up and down, whereas these waves of light or electric disturbance oscillate in all possible directions at right angles to that in which they move forward. The name applied to such electric waves is "Hertzian waves," after their discoverer, the late Professor Hertz of Bonn.

It has been found that the leaves of a book held with its edge towards the source of electric waves has the effect of polarising these waves. They are not much diminished in intensity by their passage through the book, and if a second book be held edgewise, with its leaves parallel to those of the first book, the waves can still pass on. But if the one book be turned round so that its leaves are at right angles to those of the other, the electric waves are blocked, and are no longer able to pass. The Hertzian waves are so long, and are made in such a manner, that it is possible to ascertain in which plane they are oscillating ; indeed, the experiment with the book proves this. For it is known that if the plane of the waves coincides with that of the pages of the book, the waves are annihilated ; they cause electrical currents in each leaf, and are thus absorbed. But as each leaf is separated from its neighbour by a thin layer of air, which is a practical insulator of electricity, if the leaves of the book are turned at right angles to the position in which they annihilate the electric waves, these waves cannot excite currents in the leaves, because each leaf is insulated from its neighbour, and the currents have no scope. After passage through the book, those waves which were originally oscillating in the plane of these leaves are annihilated, and used up in inciting feeble electric currents in each leaf, while those waves originally at right angles to the plane of the book's leaves pass through. Waves oscillating in intermediate planes, e.g. at an angle of 45° to the plane of the leaves, are partly annihilated, and partly pass, in proportion to the angle which they make.

The coarsely foliated structure of a book, it cannot be doubted, is analogous to the structure of a plate of tourmaline. It is also almost certainly foliated, hut the foliations are extremely minute, so that the influence they have in transmitting or obscuring light waves is commensurate with the difference in the oscillation-length of light waves and Hertzian waves. The light which passes through tourmaline, like the electric waves which pass through the leaves of a book held edgewise, have this peculiarity—they oscillate in one plane, and resemble in that respect the waves of the sea. They are said to be polarised.

Now it has been found that certain substances, such as crystals of quartz or of chlorate of potassium, have the curious property of rotating the plane of oscillation of polarised light; that is, light polarised by passage through a plate of tourmaline (or by other means, for there are more convenient plan& of polarising light),.'and Ihen transmitted through a plate of ci)stallised potassium chlorate, is not wholly obscured when it impinges on a second plate of tourmaline held at right angles to the first; it is necessary to turn the second tourmaline through more than a right angle in order that total obscuration shall result. The plane of polarisation is rotated by the chlorate crystal. If, however, the crystal is dissolved in water, its solution has no such property ; and it is inferred that the rotation is due, not to any arrangement of the atoms in the molecule of chlorate, but to the arrangement of the molecules in the crystal. For if the rotation were due to the former cause, it would be produced by solution as well as by the solid. It is believed, therefore, that the molecules in a crystal of chlorate are arranged with regard to each other like the stones in a spiral staircase.