SODIUM CHLORIDE BEBTHOLLET J 8 LA AYS 417 



weight decreases), and at a white heat it volatilises with great ease and 

 entirely; but at the ordinary temperature it may, like all ordinary 

 salts, be considered as non-volatile, although as yet no exact experi- 

 ments have been made in this respect. 



A saturated 13 solution of table salt (containing 26*4 per cent.) has 

 at the ordinary temperature a specific gravity of about l"2. The 

 specific gravity of the crystals is 2' 16. The salt which separates out 

 at the ordinary and higher temperatures contains no water of crystal- 

 lisation; 14 but if the crystals are formed at a low temperature, 

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

 a prismatic form, and contain two equivalents of water, NaCl, 2H 2 0. 

 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 



15 By combining the results of Poggiale, Miiller, and Karsten (they are evidently 

 more accurate than those of Gray-Lussac and others), I found that a saturated solution at 

 t, from to 108, contains 35'7 + 0'024!f + 0'0002j! 2 grams of salt per 100 grams of water. 

 This formula gives a solubility at = 35'7 grams ( = 26'3 p.c.), whilst according to Kar- 

 sten it is 36'09, Poggiale 35' 5, and Miiller 35'6 grams. The somewhat large diversity in 

 the data respecting so common a substance as sodium chloride shows the necessity of 

 fresh and most exact determinations. 



14 Perfectly pure fused salt is not hygroscopic, according to Karsten, whilst the 

 crystallised salt, even when quite pure, attracts as much as 0'6 p.c. of water from moist 

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

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

 -;ummer during damp weather the walls become moist, while in winter they are dry. 

 This is in accordance with the fact that the vapour tension of solutions has a definite 

 magnitude, which is less than that of water. 



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

 scopic. If it contain any magnesium chloride it partially effloresces in a damp atmosphere. 

 The crystallised and not perfectly pure salt decrepitates when heated, owing to its con- 

 taining water. The pure salt, and also the transparent rock salt, or that which has been 

 once fused, does not decrepitate. Fused sodium chloride gives a feeble alkaline reaction 

 with litmus, as has been shown by many observers, which is due to the formation 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 with even 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. 



15 By cooling a solution of table salt saturated at the ordinary temperature to 15 

 I first obtained well-formed tabular (six-sided) crystals, which on arriving at 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 what is the reason of the difference in crystalline form. It is known 

 (Mitscherlich) that NaI,2!L>O also crystallises in plates or prisms. Sodium bromide also 

 crystallises with 2HoO at the ordinary temperature. 



15 Notwithstanding the great simplicity (p. 91) of the observations on the formation 

 of ice from solution, still even for sodium chloride they cannot yet be considered as suffi- 

 ciently harmonious. According to Blagden and Raoult the temperature of the formation 

 VOL. I. BE 



