SODIUM CHLORIDE BERTHOLLET'S LAWS 427 



(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 surface of the water for 

 a long time, and sometimes go on increasing to a considerable extent, 

 but they sink directly the water penetrates inside them. Salt fuses 

 to a colourless liquid (sp. gr. 1-602, according to Quincke) at 851 

 (V. Meyer) ; if pure it solidifies to a non -crystalline mass, and if impure 

 to an opaque mass whose surface is not smooth In fusing, sodium 

 chloride commences to volatilise (its weight decreases) and at a white 

 heat it volatilises with. great ease and completely ; but at the ordinary 

 temperature it may, like all ordina/ry salts, be considered as non- volatile, 

 although as yet no exact experiments have been made in this direction. 

 A saturated 13 solution of table salt (containing 26'4 p.c.) has at 

 the ordinary temperature a specific gravity of about 1*2. The specific 

 gravity of the crystals is 2*167 (17). The salt which separates out 

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

 lisation j l4 but if the crystals are formed at a low temperature, 



12 If a solution of sodium chloride'fce slowly heated from above, where the evaporation 

 takes place, then the upper layer will become saturated before the lower and cooleif 

 layers, and therefore crystallisation will begin on the surface, and the crystals first formed 

 will float, having also dried from above, on the surface until they become 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 



7 the side of the original crystals. A funnel is formed in this manner. It will be 

 borne on the surface like a boat (if the liquid be quiescent), 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 T2004; at 15 a 

 saturated solution contains 26'5 p.c. of salt and has a sp. gr. 1-208 at 15/4. Hence a 

 solution saturated at a higher temperature is specifically lighter, notwithstanding the 

 greater amount of salt it contains. With many substances surface crystallisation cannot 

 take place 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 

 as 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, which has long been known. These circumstances, as well 

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

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

 polar and tropical climes) and at various depths. 



13 By combining the results of Poggiale, Mu'ller, and Karsten (they are evidently 

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

 ' t, from to 108, contains 85'7 + 0'024 + O'OOtm 2 grams of salt per 100 grams of water. 



This formula gives a solubility at = 85-7 grams ( = 26'3 p.c.), whilst according to Kar 

 aten it is 86*09, Poggiale 85'5, and Miiller 85'6 grams. 



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.o. of water from moist 



