Hydrochloric Acid.—­The greater part of the hydrochloric acid manufactured in Great Britain is obtained as an intermediate product in the Leblanc alkali process, which will presently be described, being produced by heating common salt with vitriol.  A large quantity is, however, also produced by the so-called direct process of Hargreaves & Robinson, which is, in principle, the same method as that employed in the Leblanc process, except that the intermediate product, vitriol, is not separated.  It consists essentially in passing the hot gases from pyrites kilns, as used in the manufacture of vitriol, through large cast-iron vessels containing common salt heated to a high temperature.  Various physical conditions must be complied with in order to make the process a success.  For example, the salt is used in the form of moulded hard porous cakes made from a damp mixture of common salt and rock salt.  The cast-iron vessels must be heated uniformly, and the hot pyrites kiln gases must be passed downwards through the salt in order to ensure uniform distribution.  The hydrochloric acid is condensed in stoneware pipes connected with towers packed with coke or stoneware.

Alkali:  Leblanc Process.—­The manufacture of vitriol, as I have described it to you, is the first step in the Leblanc process.  The next stage consists in the manufacture of sodium sulphate (salt-cake) and hydrochloric acid from the sulphuric acid and common salt; this is called the salt-cake process.  The production of salt-cake or crude sodium sulphate is carried out in two stages.  A large covered iron pan, called the decomposing pan or salt-cake pot, is mounted in one part of the salt-cake furnace, and alongside it is the hearth or bed on which the second stage of the process, the drying or roasting, is effected.  The mixture of common salt and vitriol is charged into the salt-cake pot, which is heated by a fire below.  When from two-thirds to three-quarters of the hydrochloric acid has been expelled from the charge, the mass acquires the consistence of thick dough, and at this stage it is raked out of the pan on to the roasting hearth alongside, where the decomposition is completed by means of flames playing directly on to the top of the charge.  The hydrochloric acid evolved during the process is condensed in much the same manner as in the process of Hargreaves & Robinson previously described.  It is a curious fact that in the earlier years of the Leblanc process, hydrochloric acid, or “spirits of salt,” as it is frequently called, was a by-product that required all the vigilance of the alkali-works inspectors to prevent it being allowed to escape from the chimneys in more than a certain small regulated amount.  Now, it is the principal product; indeed, the Leblanc alkali maker may be said to subsist on that hydrochloric acid, as his chief instrument for producing chloride of lime or bleaching powder.

Mechanical furnaces are now used to a large extent for the salt-cake process.  They consist broadly of a large revolving furnace-hearth or bed, on to which the mixture of salt and vitriol is charged, and on which it is continuously agitated, and gradually moved to the place of discharge, by rakes or the like, operated by suitable machinery.