The point at which neutralisation of an acid by alkali or vice versa 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 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