just takes place is ascertained very accurately by the use of certain
sensitive colours. At first litmus and cochineal tinctures were used,
but in testing crude alkalis containing alumina and iron, it was found
that lakes were formed with these colours, and they become precipitated
in the solution, and so no longer sensitive. The chemist was then
obliged to resort to certain sensitive coal-tar colours, which did not,
as the dyer and printer knew, form lakes with alumina and iron, such as
methyl orange, fluorescein, Congo red, phenolphthalein, and so forth.
For determining the alkalimetric strength of commercial sodas, a known
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