[67] Certain substances are capable of forming together only one compound, others several, and these of the most varied degrees of stability. The compounds of water are instances of this kind. In solutions the existence of several different definite compounds must be acknowledged, but many of these have not yet been obtained in a free state, and it may be that they cannot be obtained in any other but a liquid form--that is, dissolved; just as there are many undoubted definite compounds which only exist in one physical state. Among the hydrates such instances occur. The compound CO_{2} + 8H_{2}O (_see_ note 31), according to Wroblewski, only occurs in a solid form. Hydrates like H_{2}S + 12H_{2}O (De Forcrand and Villard), HBr + H_{2}O (Roozeboom), can only be accepted on the basis of a decrease of tension, but present themselves as very transient substances, incapable of existing in a stable free state. Even sulphuric acid, H_{2}SO_{4}, itself, which undoubtedly is a definite compound, fumes in a liquid form, giving off the anhydride, SO_{3}--that is, it exhibits a very unstable equilibrium. The crystallo-hydrates of chlorine, Cl_{2} + 8H_{2}O, of hydrogen sulphide, H_{2}S + 12H_{2}O (it is formed at 0°, and is completely decomposed at +1°, as then 1 vol. of water only dissolves 4 vols. of hydrogen sulphide, while at 0·1° it dissolves about 100 vols.), and of many other gases, are instances of hydrates which are very unstable. [68] Of such a kind are also other indefinite chemical compounds; for example, metallic alloys. These are solid substances or solidified solutions of metals. They also contain definite compounds, and may contain an excess of one of the metals. According to the experiments of Laurie (1888), the alloys of zinc with copper in respect to the electro-motive force in galvanic batteries behave just like zinc if the proportion of copper in the alloy does not exceed a certain percentage--that is, until a definite compound is attained--for in that case particles of free zinc are present; but if a copper surface be taken, and it be covered by only one-thousandth part of its area of zinc, then only the zinc will act in a galvanic battery.