soi)] r.M 517 



37 ; both salts, on crystallising at the ordinary temperature, combine 

 with ten molecules of water, and such crystals of soda, like crystals of 

 Glauber's salt, fuse at 34. Sodium carbonate also forms a super- 

 saturated solution, and, according to the conditions, gives various com- 

 binations with water of crystallisation (mentioned on page 107), &c. 



At a red heat superheated steam liberates carbonic anhydride from 

 sodium carbonate and forms caustic soda, Na 2 CO 3 + H. 2 O = 2NaHO4- 

 CO. 2 . Here the carbonic anhydride is replaced by water ; this depends 

 on the feebly acid character of carbonic anhydride. By direct heating, 

 sodium carbonate is only slightly decomposed into sodium oxide and 

 carbonic anhydride ; thus, when sodium carbonate is fused, about 

 1 per cent, of carbonic anhydride is disengaged. 19 The carbonates of 

 many other metals for instance, of calcium, copper, magnesium, iron, 

 <fcc. on being heated lose all their carbonic anhydride. This shows 

 the considerable basic energy which sodium possesses. With the 

 soluble salts of most metals, sodium carbonate gives precipitates 

 either of insoluble carbonates of the metals, or else of the hydroxides 

 {in this latter case carbonic anhydride is disengaged) ; for in- 

 stance, with barium salts it precipitates an insoluble barium car- 

 bonate (BaCls + Na^COg^NaCl + BaCOa) and with the aluminium 

 salts it precipitates aluminium hydroxide, carbonic anhydride being 

 disengaged : 3Na,CO 3 + A1,(SO 4 ) 3 + 3H 2 = 3Na 2 S0 4 + 2A1(OH) 3 + 

 3C0. 2 . Sodium carbonate, like all the salts of carbonic acid, evolves 

 carbonic anhydride on treatment with all acids which are to any extent 

 energetic. But if an acid diluted with water be gradually added to a 

 solution of sodium carbonate, at first such an evolution does not take 

 place, because the excess of the carbonic anhydride forms acid sodium 

 carbonate (sodium bicarbonate), NaHCO 3 . The composition of this 

 salt, however, may be also represented as a combination of car- 

 bonic acid, H. 2 C0 3 , with the normal salt, Na. 2 CO 3 , just as the latter 

 also combines with water. Such a representation is all the more 



the formula, s = 9092 + 104-5^ + 0-165^. Weak solutions occupy a volume not only less 

 than the sum of the volumes of the anhydrous salt and the water, but even less than the 

 water contained in them. For instance, 1000 grams of 1 p.c. solution occupy (15 C ) a 

 volume of 990'4 c.c. (sp. gr. 1'0097), but contain 900 grams of water, occupying at 15 a 

 volume of 990'8 c.c. A similar case of solubility, which is comparatively rare (it occurs 

 also with sodium hydroxide), occurs in those dilute solutions for which the factor A is 

 greater than 100 if the sp. gr. of water at 4 = 100000, and if the sp. gr. of the solution be 

 expressed by the formula S= S + Ap + Bp 2 , where S is the specific gravity of the water 

 (this is more fully discussed in my treatise, The Investigation of Aqueous Solutions, 

 1887, 94 and 95). For 5 p.c. the sp. gr. 15/4 c = r0520 ; for 10 p.c. T1057 ; for 15 p.c. 

 1*1603. The changes in the sp. gr. with the temperature are here almost the same as 

 with solutions of sodium chloride with an equal value of p. 



19 According to the observations of Pickering. According to Rose, when solutions of 

 sodium carbonate are boiled a certain amount of carbonic anhydride is disengaged. 



