420 PRINCIPLES OF CHEMISTRY 



It should be remarked that Baume's hydrometer is graduated by 

 taking a 10 per cent, solution of sodium chloride as 10 on the scale, 

 and therefore it gives approximately the percentage amount of the salt 

 in a solution. Table salt is somewhat soluble in alcohol, 21 but it is 

 insoluble in ether and in oils. 



Table salt gives very few compounds 22 (double salts), and these are 

 very readily decomposed ; it is also decomposed with great difficulty 

 and its dissociation is unknown. 23 But it is easily decomposed, both 

 when fused and in solution, by the action of a galvanic current. If the 

 dry salt be fused in a crucible and an electric current be passed through 

 it by immersing carbon or platinum electrodes in it (the positive elec- 

 trode is made of carbon and the negative of platinum or mercury), it is 

 decomposed into two substances : a malodorous gas called chlorine appears 

 at the positive pole and metallic sodium at the negative pole, which 

 shows that table salt consists of these two elements. Both of them act 

 on water at the moment of their evolution ; the sodium, as we already 

 know, evolves hydrogen from water and forms caustic soda, and the 

 chlorine evolves oxygen from water and forms hydrochloric acid, and 

 therefore on passing a current through a solution of table salt metallic 

 sodium will not be formed but oxygen, chlorine, and hydrochloric acid 

 will appear at the positive pole, and hydrogen and caustic soda at the 

 negative pole. The presence of hydrochloric acid is easily recognised 

 by its acid properties, and the presence of caustic soda by its alkaline 



weight of water at the same temperature and in the same volume, then the true sp. gr. 

 S referred to water at 4 is found by multiplying S 2 by the sp. gr. of water at the tem- 

 perature of observation. All the necessary corrections for the specific gravity of liquids 

 are considered in my two works, On the Compounds of Alcohol with Water, 1865, and 

 The Investigation of Aqueous Solutions by their Specific Gravity, 1887. 



It may not be superfluous to remark that the data respecting the sp. gr. for solutions 

 of sodium chloride near saturation do not sufficiently agree, and there is reason for 

 thinking that in strong solutions containing more sodium chloride than in NaCl,10H.,O 

 (^j = 24'58) a different curve should be adopted. 



21 According to Schiff 100 grams of alcohol, containing p p.c. by weight of C 2 H 6 O, 

 dissolves at 15 



_p = 10 20 40 60 80 



25 22'6 13-2 5'9 1'2 grams NaCl. 



22 Amongst the double salts formed by sodium chloride that obtained by Ditte (1870) 

 by the evaporation of the solution remaining after heating sodium iodate with hydro- 

 chloric acid until chlorine ceases to be liberated, is a remarkable one. Its composition is 

 NaIO3,NaCl,4H 2 O. Rammelsberg obtained a similar (perhaps the same) salt in well- 

 formed crystals by the direct reaction of both salts. 



23 But it already gives sodium in the flame of a Bunsen's burner (see Spectrum 

 Analysis), doubtless under the reducing action of the elements carbon and hydrogen. In 

 the presence of an excess of hydrochloric acid in the flame (when the sodium would give 

 sodium chloride), there is no sodium formad in the flame and the salt does not communi- 

 cate its usual coloration. 



