~Historical.~ Water was long regarded as an element. In 1781 Cavendish showed that it is formed by the union of hydrogen and oxygen. Being a believer in the phlogiston theory, however, he failed to interpret his results correctly. A few years later Lavoisier repeated Cavendish's experiments and showed that water must be regarded as a compound of hydrogen and oxygen. ~General methods employed for the determination of the composition of a compound.~ The composition of a compound may be determined by either of two general processes these are known as _analysis_ and _synthesis_. 1. _Analysis_ is the process of decomposing a compound into its constituents and determining what these constituents are. The analysis is _qualitative_ when it results in merely determining what elements compose the compound; it is _quantitative_ when the exact percentage of each constituent is determined. Qualitative analysis must therefore precede quantitative analysis, for it must be known what elements, are in a compound before a method can be devised for determining exactly how much of each is present. 2. _Synthesis_ is the process of forming a compound from its constituent parts. It is therefore the reverse of analysis. Like analysis, it may be either qualitative or quantitative. ~Application of these methods to the determination of the composition of water.~ The determination of the composition of water is a matter of great interest not only because of the importance of the compound but also because the methods employed illustrate the general methods of analysis and synthesis. ~Methods based on analysis.~ The methods based on analysis may be either qualitative or quantitative in character. [Illustration: Fig. 18]