416 PRINCIPLES OF CHEMISTRY 



salt always crystallises in the cubic system, most frequently in cubes, 

 and more rarely in octahedra. Large transparent cubes of common salt, 

 having edges up to 10 centimetres long, are sometimes found in masses 

 of rock salt. 11 By rapid evaporation common salt is deposited from its 

 solutions in the form of very small crystals, but with slow evaporation 

 the crystals attain a considerable size. When evaporated in open 

 spacer the salt often separates out on the surface 12 as cubes, which 

 grow on to each other in the form of pyramidal square funnels. In 

 still weather, these clusters are able to support themselves on the sur- 

 face of the water for a long time, and sometimes go on increasing to a 

 considerable degree, but they sink directly the water penetrates inside 

 them. Salt fuses into a colourless liquid (sp. gr. 1*602, according to 

 Quincke) at about 774 (Carnelley), and if pure it solidifies into a non- 

 crystalline mass, and if impure into an opaque mass whose surface is 

 not smooth. In fusing, sodium chloride commences to volatilise (its 



In this respect salt is twice as secure as bricks, and therefore immense masses may be 

 "extracted from underground workings with perfect safety, without having recourse 

 to brickwork supports, but only taking advantage of the properties of the salt itself. 



n To obtain well-formed crystals, a saturated solution is mixed with ferric chloride, 

 and several small crystals of sodium chloride are placed at the bottom, and the solution 

 is allowed to slowly evaporate in a closed vessel. Octahedral crystals are obtained in the 

 presence of borax, urea, &c., in the solution. Very fine crystals are formed in a mass of 

 gelatinous silica. 



12 If a solution of sodium chloride be slowly heated from above, where the evapora- 

 tion is accomplished, then the upper layer will become saturated before the lower and 

 cooler layers, and therefore crystallisation will then begin on the surface, and the crystals 

 first formed will be held up, having also dried from above, on the surface until they be- 

 come quite soaked. Being heavier than the solution the crystals are partially immersed 

 under it, and the following crystallisation, also proceeding on the surface, will only form 

 crystals along the side of the original crystals. The funnels are formed in this manner. 

 It will be borne on the surface like a boat (if there be no waves), because it will grow 

 more from the upper edges. We can thus understand this, at first sight, strange funnel 

 form of crystallisation of salt. In explanation why the crystallisation under the above 

 conditions begins at the surface and not at the lower layers, it must be mentioned that 

 the specific gravity of a crystal of sodium chloride = 2'16, and that of a solution saturated 

 at 25 contains 26'7 p.c. of salt and has a specific gravity at 25'4 of 1-2004 ; at 15 a 

 solution contains 26'5 p.c. of salt and has a sp. gr. 1'203 at 15'4. Hence a solution satu- 

 rated at a higher temperature is specifically lighter, notwithstanding the greater amount 

 of salt it contains. Surface crystallisation cannot take place with many substances, 

 because their solubility increases more rapidly with the temperature than their specific 

 gravity decreases. In this case the saturated solution will always be in the lower layers, 

 where also the crystallisation will take place. Besides which, it may be added that, us a 

 consequence of the properties of water and solutions, when they are heated from above 

 (for instance, by the sun's rays) the warmer layers being the lightest remain above, whilst 

 when heated from below they rise to the top. For this reason the water at great depths 

 below the surface is always cold, as has long been known. These circumstances, as well 

 &s those observed by Soret (Chapter I. Note 19), explain the great differences of density, 

 temperature, and in the amount of salts held in the oceans at different latitudes (in polar 

 and tropical climes) and at various depths. 



