In such a proposition as _Salt dissolves_, the copula (_is_) is contained in the predicate, and, besides the subject, only one element is exhibited: it is therefore said to be _secundi adjacentis_. When all three parts are exhibited, as in _Salt is soluble_, the proposition is said to be _tertii adjacentis_. For the ordinary purposes of Logic, in predicating attributes of a thing or class of things, the copula _is_, or _is not_, sufficiently represents the relation of subject and predicate; but when it is desirable to realise fully the nature of the relation involved, it may be better to use a more explicit form. Instead of saying _Salt--is--soluble_, we may say _Solubility--coinheres with--the nature of salt_, or _The putting of salt in water--is a cause of--its dissolving_: thus expanding the copula into a full expression of the relation we have in view, whether coinherence or causation. § 3. The sentences of ordinary discourse are, indeed, for the most part, longer and more complicated than the logical form of propositions; it is in order to prove them, or to use them in the proof of other propositions, that they are in Logic reduced as nearly as possible to such simple but explicit expressions as the above (_tertii adjacentis_). A Compound Proposition, reducible to two or more simple ones, is said to be exponible. The modes of compounding sentences are explained in every grammar-book. One of the commonest forms is the copulative, such as _Salt is both savoury and wholesome_, equivalent to two simple propositions: _Salt is savoury; Salt is wholesome. Pure water is neither sapid nor odorous_, equivalent to _Water is not sapid; Water is not odorous_. Or, again, _Tobacco is injurious, but not when used in moderation_, equivalent to _Much tobacco is injurious; a little is not_. Another form of Exponible is the Exceptive, as _Kladderadatsch is published daily, except on week-days_, equivalent to _Kladderadatsch is published on Sunday; it is not published any other day_. Still another Exponible is the Exclusive, as _Only men use fire_, equivalent to _Men are users of fire; No other animals are_. Exceptive and exclusive sentences are, however, equivalent forms; for we may say, _Kladderadatsch is published only on Sunday_; and _No animals use fire, except men_. There are other compound sentences that are not exponible, since, though they contain two or more verbal clauses, the construction shows that these are inseparable. Thus, _If cats are scarce, mice are plentiful_, contains two verbal clauses; but _if cats are scarce_ is conditional, not indicative; and _mice are plentiful_ is subject to the condition that _cats are scarce_. Hence the whole sentence is called a Conditional Proposition. For the various forms of Conditional Propositions see chap. v. § 4.