In the systems which have so far been studied, we have met with cases where two or three components could enter into combination; but in no case did we find double decomposition occurring. The reason of this is that in the systems previously studied, in which double decomposition might have been possible, namely in those systems in which two salts acted as components, the restriction was imposed that either the basic or the acid constituent of these salts must be the same; a restriction imposed, indeed, for the very purpose of excluding double decomposition. Now, however, we shall allow this restriction to fall, thereby extending the range of study. Hitherto, in connection with four-component systems, the attention has been directed solely to the study of aqueous solutions of salts, and more especially of the salts which occur in sea-water, _i.e._ chiefly, the sulphates and chlorides of magnesium, potassium, and sodium. The importance of these investigations will be recognized when one recollects that by the evaporation of sea-water there have been formed the enormous salt-beds at Stassfurt, which constitute at present the chief source of the sulphates and chlorides of magnesium and potassium. The investigations, therefore, are not only of great geological interest as tending to elucidate the conditions under which these salt-beds have been formed, but are of no less importance for the industrial working of the deposits. It is, however, not the intention to enter here into any detailed description of the different systems which have so far been studied, and of the sometimes very complex relationships {312} met with, but merely to refer briefly to some points of more general import in connection with these systems.[382] Reciprocal Salt-Pairs. Choice of Components.--When two salts undergo double decomposition, the interaction can be expressed by an equation such as NH_{4}Cl + NaNO_{3} = NaCl + NH_{4}NO_{3} Since one pair of salts--NaCl + NH_{4}NO_{3}--is formed from the other pair--NH_{4}Cl + NaNO_{3}--by double decomposition, the two pairs of salts are known as _reciprocal salt-pairs_.[383] It is with systems in which the component salts form reciprocal salt-pairs that we have to deal here.