740 



SALT 



Of 100 rays of heat 



Clear rock-salt transmits 

 Muddy ditto 



Plate glass 



Clear ice 



92 



65 



24 







The source of heat in these experiments was red-hot platinum. 



Chloride of sodium occurs in nature chiefly in two forms, either as rock-salt, form- 

 ing extensive deposits, or disseminated in minute quantity through the mass of the 

 strata which form the earth's crust. Water penetrating the layers of rock-salt, and 

 exerting there a solvent action, gives rise to the brine springs which are found in 

 various countries; whilst streams and rivers dissolving the same substance out of the 

 strata through which they flow, carry it down to the sea, where it constitutes the 

 principal saline ingredient in the waters of the ocean. 



Even in mass, as rock-salt (Sel gemme, Fr. ; Steinscdz, Ger.), this substance 

 possesses a crystalline structure derived from the cube, which is its primitive form. 

 It has generally a foliated texture, and a distinct cleavage, but it has also sometimes 

 a fibrous structure. Its lustre is vitreous, and its streak white. It is not so brittle 

 as nitre; its hardness = 2-5, which is nearly that of alum; a little harder than 

 gypsum, but softer than calcareous spar. Its specific gravity varies between 2-1 and 

 2-257. It is white, occasionally colourless, and perfectly transparent, but usually of 

 a yellow or red, and more rarely of a blue or purple tinge. A few analyses will show 

 the general purity of this substance. 



The principal impurities occurring in rock-salt are sulphate of lime, oxide of iron 

 and clay, but the chlorides of potassium, calcium, and magnesium, the sulphates of 

 soda and magnesia, and bituminous matters, are sometimes found in it; and occasion- 

 ally shells, and insect and infusorial remains, exist enclosed in the mass. To the 

 presence of infusoria, indeed, is attributed the red or green colour with which some 

 varieties are tinted, which, upon analysis, are found to be absolutely pure chloride of 

 sodium, as in the case of the second specimen quoted in the above table. Carburetted 

 hydrogen gas in a state of strong compression is' met with in some varieties, and 

 these when dissolved in water emit a peculiar crackling sound, caused by the 

 expansion and escape of the confined gas. 



The geological position of rock-salt is very variable ; it is found in all sedimentary 

 formations, from the palaeozoic to the tertiary, and is generally interstratified with 

 gypsum, and associated with beds of clay. When the latter is present in largo 

 quantity, the term ' saliferous clay ' is applied to the deposit. The great British 

 deposits of salt in Cheshire and Worcestershire are found in the New Red Sandstone. 

 At Northwich, in the Vale of the Weaver, the rock-salt consists of two beds, which are 

 not less than 100 feet thick, and are supposed to constitute large insulated masses, 

 about a mile and a half long, and nearly 1,300 yards broad. There are other deposits 

 of rock-salt in the same valley, but of inferior importance. The uppermost bed occurs 

 at 75 feet beneath the surface, and is covered with many layers of indurated red, blue, 

 and brown clay, interstratified more or less with gypsum, and interspersed with argil- 

 laceous marl. The second bed of rock-salt lies at 31 feet below the first, being sepa- 

 rated from it by layers of indurated clay, with veins of rock-salt running between them. 

 The lowest bed of salt was excavated to a depth of 1 10 feet, several years ago. Many 

 of the German deposits of rock-salt occur in their Hunter Sandstein, which is the re- 

 presentative of part of our New Red Sandstone, and is so called because its colours vary 

 from red to salmon and chocolate. In the Austrian Alps salt is found in oolitic lime- 

 stone ; at Cardonna, in Spain, in the greensand ; and the famous mines of Wieliczka, in 

 Galicia (excavated at a depth of 860 feet, in a layer 500 miles long, 20 broad, and 



