weight of the sample is dissolved in water, and a few drops of a
solution of methyl orange are added, which colour the solution yellow or
orange. Into this solution is then run, from a burette or graduated
tube, a standard solution of an acid, that is, a solution prepared by
dissolving a known weight of an acid, say hydrochloric acid, in a known
volume of water. The acid is run in gradually until the yellow colour
changes to pink, at which point the volume of acid used is noted.
Knowing the weight of acid contained in this volume of standard acid,
and having regard to the law of equivalence mentioned above, it is an
easy matter to calculate the amount of alkali equivalent to the acid
used, and from this the alkali contained in the sample.

_Sulphuric Acid._--The first process for manufacturing sulphuric acid or
vitriol was by placing some burning sulphur in a closed vessel
containing some water. The water absorbed the acid formed by the burning
sulphur. It was next discovered that by mixing with the sulphur some
nitre, much more sulphuric acid could be produced per given quantity of
brimstone. At first large glass carboys were used, but in 1746 the
carboys were replaced by chambers of lead containing water at the
bottom, and in these lead chambers the mixture of sulphur and nitre was
burnt on iron trays. Next, although gradually, the plant was divided
into two portions--a furnace for burning the sulphur, and a chamber for
receiving the vapours. The system was thus developed into the one
followed at the present time. The sulphur, or, in most cases, cupreous
iron pyrites (a combination of iron and copper with sulphur), is burned
in specially constructed kilns or furnaces, and the hot gases,
consisting essentially of sulphur dioxide with the excess of air, pass
through flues in which are placed cast-iron "nitre pots" containing a
mixture of nitre (sodium nitrate) and vitriol. The gases thus become
mixed with nitrous fumes or gaseous oxides of nitrogen, and, after
cooling, are ready for mixing with steam or water spray in the lead
chambers in which the vitriol is produced. These oxides of nitrogen
enable the formation of sulphuric acid to take place more quickly by
playing the part of oxygen-carriers. Sulphuric acid is formed by the
union of oxygen with sulphur dioxide and water; the oxides of nitrogen
combine with the oxygen of the air present in the chambers, then give up
this oxygen to the sulphur dioxide and water or s