In 1779 three beds of rock-salt were discovered at Lawton, separated from one another by layers of indurated clay.
Halite or rock-salt crystallizes in the cubic system, usually in cubes, rarely in octahedra; the cubes being solid, unlike the skeleton-cubes obtained by rapid evaporation of brine.
Glauber showed how to prepare hydrochloric acid, spiritus salis, by heating rock-salt with sulphuric acid, the method in common use to-day; and also nitric acid from saltpetre and arsenic trioxide.
One great drawback to the use of even the purest rock-salt simply ground is its tendency to revert to a hard unwieldy mass, when kept any length of time in sacks.
The development has been mainly since 1887 at Hutchinson and since about 1890 in the rock-salt mines.
In England extensive deposits of rock-salt are found near the base of the Keuper marl, especially in Cheshire.
The purer rock-salt is often simply ground for use, as at Wieliczka and elsewhere, but it is more frequently pumped as brine, produced either by artificial solution as at Middlesbrough and other places, or by natural means, as in Cheshire and Worcestershire.
One difference between the manufacture of salt from rock-salt brine as carried on in Britain and on the Continent lies in the use in the latter case of closed or covered pans, except in the making of fine salt, whereas in Britain open ones are employed.
As usually made, white salt from rock-salt may be classified into two groups: (I) boiled: known as fine, table, lump, stoved lump, superfine, basket, butter and cheese salt (Fr.
Chlorine is never found in nature in the uncombined condition, but in combination with the alkali metals it occurs widely distributed in the form of rock-salt (sodium chloride); as sylvine and carnallite, at Stassfiirt; and to a smaller extent in various other minerals such as matlockite and horn-mercury.