324 



DISCOVERY 



salt is twice as strong as bricks in compression. It 

 will be readily understood that no water whatever has 

 any access into the workings, which attain the con- 

 siderable depth of 12,000 ft. below the snrfacc. In the 

 past disasters have occurred owing to the influx 

 of underground waters into the mines. 



In England, down to 1850 most of the rock-salt 

 deposits in Cheshire and Worcestershire were mined ; 

 as such mines one by one became flooded, brine- 

 pumping was resorted to, and at the present time, salt 

 in England is almost entirely obtained from brine. The 

 overlying clays are so punctured that fresh surface- 

 water has now had access to the beds of rock-salt, in 

 which are sunk several bore-holes. The brine is 

 usually lifted by an air-lift which forces compressed air 

 to the bottom of the bore-hole ; then the force of the 

 compressed air rising to the surface lifts the brine. 

 Fresh water probably finds its way through salt 

 deposits in underground channels which are con- 

 tinually being enlarged owing to the dissolution of the 

 salt by the fresh water travelling through them ; 

 occasionally the brine appears to be over-pumped, 

 i.e. its level in the bore-hole sinks considerably ; this 

 is attributed to the underground channel in the 

 rock-salt being temporarily blocked by a coOapse of 

 the clay-roof above the rock-salt. But when the 

 brine-level in the bore-hole is lowered, a greater 

 hydrostatic pressure is put on the brine, which thereby 

 usually cuts another way through to the shaft and 

 so returns to its normal level. Naturally, considerable 

 damage occurs at the surface in subsidences with all 

 this disintegration and dissolution going on imder- 

 ground, at a depth of not very much more (in many 

 places) than 150 ft., and damage to property in the 

 district occurs from time to time. 



Manufacture of Salt 



There are several processes of extracting the salt 

 from the brine ; one very recently adopted is known as 

 the Hodgkinson process. In this the brine is pumped 

 into a large enclosed pan, 30 ft. in diameter, which is 

 heated by the hot gases from a mechanically stoked 

 furnace. The temperature of the hot gases is so 

 controlled that the crystals of salt can be varied in 

 size ; either very small crj/stals can be made for 

 table-salt or large lumps for salting and packing fish. 

 There are two other smaller secondary pans in which 

 table-salt is produced, and the gases pass on to four 

 large rectangiflar open pans 60 ft. by 25 ft. where 

 coarse crystals only are made, a slower rate of evapora- 

 tion and crystallisation taking place in these pans. 

 By this process about 7 tons of salt are produced to 

 every ton of coal consumed, and the finest table salt 

 is produced without grinding the larger crystals. In 

 the older process (of evaporating brine in open pans with 



the furnace in contact with the pans), only 2 tons of 

 salt were produced per ton of coal, and finer varieties 

 of salt had to be obtained by grinding the coarser 

 crystals. The extraction of the salt by this process 

 and the use of the air-lift in pumping the brine, instead 

 of the expensive method of quarrying rock-salt, has 

 revolutionised the industry. 



Rock-salt and Public Water-supplies 



It must be said, however, that the quarrying of 

 rock-salt, providing that the workings could have been 

 kept dry at all times, had the great advantage of being 

 under control, the roofs being propped up and not 

 allowed to fall in. \\'ith brine-pumping, subsidences 

 occur, but nobody knows where or when they are 

 going to take place. The whole of the top-water in the 

 strata of the district is becoming, to say the least, 

 brackish. Wells have to be abandoned and both 

 public and private water-supplies are becoming affected. 

 In the immediate future big problems may have to be 

 solved by local authorities, to cope with the question of 

 water-supply, since several large towns have wells in 

 proximity to this salt region. Moreover, brine may 

 travel many miles through the adjacent porous sand- 

 stones of the Keuper, Bunter, and strata of more or less 

 porous deposits of glacial-drift-gravel which now fill 

 up pre-glacial valleys. Such brine or brackish waters 

 may appear again in most unexpected places. It may 

 be noted that Sodium Chloride, which is so very easily 

 dissolved by water, is exceedingly difficult to extract 

 from water, and it is only by evaporation that it can 

 be so extracted. This would be quite impracticable 

 in any water-works ; even on board ship it takes i lb. 

 of coal to render a gallon of sea-water fit for drinking 

 by repeated evaporation and condensing, and even 

 then it is not very palatable. 



Alkali 



In some places, therefore, bore-holes have been sunk 

 some distance away from the rock-salt beds, by the 

 makers of alkali. In other places brine is conducted 

 in pipes, as from the Northwich district to the alkali- 

 makers at Runcorn. Salt is necessary for the manu- 

 facture of washing-soda (which in turn is used for the 

 manufacture of soap and many alkalis) either by the 

 Leblanc or the ammonia-soda process. In the for- 

 mer, brine is treated with Sulphuric Acid, and thus 

 Sodium Sulphate is formed. This substance is con- 

 verted into Sodium Carbonate, or washing-soda, by 

 being treated with Carbon Dioxide and lime. In the 

 ammonia-soda process (which is cheaper), ammonia is 

 added to brine, which is filtered and cooled and passed 

 into towers and Carbon Dioxide is caused to bubble 

 through them. The crystals of washing-soda which 

 form arc dried by heat, while the by-product. Ammo- 



