[53] Solution-pressure and osmotic pressure are really two names for the same force; the former being employed to signify the tendency of a solid to dissolve, and the latter the pressure produced by the dissolved body which tends to prevent further solution. Thus, in a saturated solution in contact with its solid, the two pressures are always equal, but exerted in opposite directions. A curious apparent deviation from this law is however noticed in solutions of salts, acids, and alkalies, and indeed of electrolytes generally; thus a dilute solution of sodium chloride produces an osmotic pressure nearly double that corresponding to the number of molecules of NaCl present; and in fact behaves as if it were a solution of Na and Cl existing separately. Such a solution conducts a current of electricity very readily, while at the same time the chlorine is carried to the positive, and the sodium to the negative pole, where they separate as Na₂ and Cl₂ (the Na decomposing the water present and forming NaOH). In fact, the modern theory of electrolysis asserts that these dissociated atoms are not separated from each other by electricity, but that they exist already separated in the solution of the electrolyte, and merely act as carriers for the electricity, and that the work done by the latter is not that of breaking up the salt-molecule, but of giving its dissociated atoms fresh charges of electricity which enable them to combine as new molecules, and escape from the electrolyte. Complex salts do not always break up into single atoms, thus calcium sulphate dissociates into Ca and SO₄, hydrogen sulphate (sulphuric acid) into 2H and SO₄, and so on. These dissociated atoms and atom-groups are called “ions,” and may be monovalent, divalent, and so on; the divalent ion carrying double the electrical quantity or charge of the monovalent. Without discussing the ultimate nature of electricity itself, the matter is most easily pictured by assuming that the molecule of the undissolved salt is made up of an ion with a + charge (“kation,” e.g. Na), and an ion with a - charge (“anion,” e.g. Cl), by the electrical attraction of which charges they are held together. In the solution these attractions are balanced by those of other ions, so that they can wander freely within the liquid, but in order to take the molecular form of free elements and escape, say as Na₂ and Cl₂, the pair of kations must go to the - pole and give up one + charge, and at the same time a pair of anions must go to the + pole and receive a + charge. Thus the Na and all other kations separate at the - pole, and the Cl and all other anions at the + pole.