rocesses 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]

1. _Qualitative analysis._ As was stated in the study of oxygen, water
may be separated into its component parts by means of the electric
current. The form of apparatus ordinarily used for effecting this
analysis is shown in Fig. 18. A platinum wire, to the end of which is
attached a small piece of platinum foil (about 15 mm. by 25 mm.), is
fused through each of the tubes B and D, as shown in the figure. The
stopcocks at the ends of these tubes are opened and water, to which has
been added about one tenth of its volume of sulphuric acid, is poured
into the tube A until the side tubes B and D are completely
filled. The stopcocks are then closed. The platinum wires extending into
the tubes B and D are now connected with the wires leading from two
or three dichromate cells joined in series. The pieces of platinum foil
within the tubes thus become the electrodes, and the current flows from
one to the other through the acidulated water. As soon as the current
passes, bubbles of gas rise from each of the electrodes and collect in
the upper part of the tubes. The gas rising from the negative electrode
is found to be hydrogen, while that from the positive electrode is
oxygen. It will be seen that the volume of the hydrogen is approximately
doub