MUR
MUR
( ns )
whi'ft the increafing bulk of the mafs caufes itgradually to fink lower in the liquor.
Common fait is very foluble in water, 8 partsof the latter will diffblve i'omcwhat lefs than3 of fait at a moderate temperature, and fcarcelyany more is taken up at a boiling heat, fo thatnet fait can be obtained from a hot faturatedfelutmn by mere cooling, but only by evapora-tion ot the fluid. Salt contains but littie waterof eryilallization, and hence when thrown inthe fire or fuddenly heated it decrepitates orflies to pieces with a crackling noife. If heatedred hot it melts, and at a ftill higher tempera-ture it flics off in the form of denfe white fumes,but is not decompofed- by volatilization.
It is compofed according to Kirwan, of ^3per cent, of foda, and 47 muriatic acid andwater, and the quantity of real acid is fuchthat 100 parts of the crystallized l'alt decom-pofed by nitrat of fiiver, will give 2337 of lunacornea, 1 of" which the mere acid is 38.6. b Itmuft be obferved that there is fome differencein the analyfes of different chemifts, but takingthe above data a* correct, too parts of cryftal-lized muriat of foda will contain 53 of foda,38.6 of acid, and 8.4 of water.
Common fait is generally faid to be infolublein alcohol, however Mr. Kirwan found that ina moderate temperature too grs. of alcohol, at.900 fp. gr. diflolved 5.8 grs. of fait ; the famequantity at .872 fp. gr. diflolves 3 .6 grs; andeven at .834 fp. gr. it diflolved a grain.
Muriat of foda may be decompofed in avariety of ways : its acid is readily and totallyexpelled by the fulphuric at a moderate heat:its alkali may be procured by a variety of me-thods, many of which are ufed in the greatway, and will be Ihortly defcribed at the endof this article.
We now proceed to the mention of methodsby which are procured the immenfe quantitiesof common fait employed by man in almoftevery country on the face of the globe.
It has been already mentioned that native faitis found, either folid, under the fur.face of theearth (when it is often termed fal gem, and ofwhich that in the county of Chefter in this countryis a ftriking example) or diflolved in naturalbrine fprings, which are always found in theneighbourhood of rock fait: or it is left by thefpontaneous evaporation of many inland lakesand pools in different parts of the world: orlallly, an inexhaultible itore of it is containedin the waters of the ocean. Sometimes the: rock fait is found fufficiently pure to be ufed
without any preparation; this however is rare,for by far the greater part of the fait ufedthroughout the globe, is got by evaporation offait water, either by natural or artificial heat,or often by both. The general procefs ofmaking fait from brine by artificial heat, is veryfimple and obvious, being little elfe than puttingthe brine into a broad fhallow iron pan, bring-ing it to a boiling heat by furnaces underneath,and continuing the evaporation nearly to dry-ncfs, during which the fait gradually feparatesas the water is diffipated, and is afterwards col-lected and gently dried. But there are l'everalcircumitances relating to falt-making which de-ferve the attention of the chemift, and requireto be related more at large.
We Ihall give in a few words the procefs offalt-making as pradtifed in Chefhire, being theplace whence mofl of the fait ufed in this countryis obtained. c
The brine is firft pumped up from very deepwells, by powerful machinery, and is difehargedin a large pond or refervoir. If the brine is weak infait, it is generally flrengthened and nearly fatu-rated by throwing in a quantity of the moreimpure rock fait dug up in the neighbourhood,particularly in thofe falt-works that have theconvenience of water-carriage from the pits.There is a -confiderable difference in the purityof the brine from different pits, all contain afmall portion of earthy fait, chiefly fulphat oflime, and a fmall quantity of carbonat of limeheld in folution by an excefs of carbonic acid,and frequently aii'o a little carbonat of iron.The pureft brine is perfectly limpid, of a purefaline tafle, and a peculiar cold green hue.This laft indicates the abfence of iron, for wheneven the fmallefl admixture of oxyd of iron isprefent, the water has a yellowilh call, and thefait made from it never acquires that delicateblue-whitenefs which is confidered as a criterionof its perfection. The falt-pans where the brineis boiled down are oblong fhallow troughs ofwrought iron, ufually from 20 to 30. feet longand broad, and about 9 to 12 inches deep.They are fet ftrongly upon mafonry, over alarge furnace, the flues of which draw all roundthe pan. The fuel is coal, of which there aremany pits at no great distance. Each panftands in a fmall covered building, with a pyra-midal roof formed of boards {loping downwards,but with a confiderable interval between each,fo as.to keep off the rain, and at the fame timeto allow of a free paflage for the fleam of theboiling brine.