428 PRINCIPLES OF CHEMISTRY 



especially from a saturated solution cooled to 12 6 , then they present 

 a prismatic form, and contain two equivalents of water, NaCl,2H 2 O. 

 At the ordinary temperature these crystals split up into sodium 

 chloride and its solution. 15 Unsaturated solutions of table salt when 

 cooled below give 16 crystals of ice, but when the solution has a 

 composition NaCl,10H 2 it solidifies completely at a temperature of 

 23 A solution of table salt saturated at its boiling point boils at 

 about 109, and contains about 42 parts of salt per 100 parts of water. 

 Of all its physical properties the specific gravity of solutions of 

 sodium chloride is the one which has been the most fully investigated. 

 A comparison of all the existing determinations of the specific gravity 



air, according to Stas. (In the Briansk mines, where the temperature throughout the 

 whole year is about + 10 3 , it may be observed, as Baron Klodt informed me, that in the 

 summer during damp weather the walls become moist, while in winter they are dry). 



If the salt contain impurities such as magnesium sulphate, &c. it is more hygro- 

 scopic. If it contain any magnesium chloride, it partially deliquesces in a damp atmo- 

 sphere. The crystallised and not perfectly pure salt] decrepitates when heated, owing to 

 its containing water. The pure salt, and also the transparent rock salCj. or that which 

 has been once fused, does not decrepitate. Fused sodium chloride shows a faint alkaline 

 reaction to litmus, which has been noticed by many observers, and is due to the presence 

 of sodium oxide (probably by the action of the oxygen of the atmosphere). According 

 to A. Stcherbakoff very sensitive "litmus (washed in alcohol and neutralised with oxalic 

 acid) shows an alkaline reaction even with the crystallised salt. 



It may be observed that rock salt sometimes contains cavities filled with a colourless 

 liquid. Certain kinds of rock salt emit an odour like that of hydrocarbons. These 

 phenomena have as yet received very little attention. 



K By cooling a solution of table salt saturated at the ordinary te"mperature to 16, 

 I obtained first of all well-formed tabular (six-sided) crystals, which when warmed to 

 the ordinary temperature disintegrated (with the separation of anhydrous sodium 

 chloride), and then prismatic needles up to 20 mm. long were formed from the same- 

 solution. I have not yet investigated the reason of the difference in crystalline form. 

 It is known (Mitscherlich) that NaI,2H 2 O also crystallises either in plates or prisms 

 Sodium bromide also crystallises with 2H 4 O at the ojrdinary temperature. 



le Notwithstanding the great simplicity (Chapter I., Note 49) of the observations on the 

 formation of ice from solution, still even for sodium chloride they cannot yet be con- 

 sidered as sufficiently harmonious. According to Blagden and Raoult, the temperature 

 of the formation of ice from a solution containing c grams of salt per 100 grams of water 

 = -0'6c to c = 10, according to Bosetti = -0'649c to c = 8'7, according to De Coppet 

 (to c = 10)= -0-55 c - 0'006c 2 , according to Karsten (to c = 10) -0-762c + 0-0084c ? , and 

 according to Guthrie a much lower figure. By taking Bosetti's figure and applying 

 the rule given in Chapter I., Note 49 we obtain 



* = 0-649 x ^ = 2-05. 



Pickering (1898) gives for c=l -0-608, for c = 2-T220; that is (c up to 2'7) about 

 - (0-600 + 0-005c)c. 



The data for strong solutions are not less contradictory. Thus with 20 .c. of salt, ice 

 is formed at -14'4 accprding to Karsten, -17 according to Guthrie, -17*6 according 

 to De Coppet. Biidorff states that for strong solutions the temperature of the formation 

 of ice descends in proportion to the contents of the compound, NaCl,2H 2 O (per 100 grams 

 of water) by 0'S42 per 1 gram of salt, and De Coppet shows that there is no proportion 

 ality, in a strict sense, for either a percentage of NaCl or of NaCl,2H 2 O. 



