516 PRINCIPLES OF CHEMISTRY 



and the decahydrated salt contains 78-9 of the anhydrous salt com- 

 bined with 100 parts of water. From the above figures it is seen that 

 the decahyd rated salt cannot fuse without decomposing, 7 like hydrate of 

 chlorine, C1 2 ,SH 2 (Chapter XI., Note 10). Not only the fused deca- 

 hydrated salt, but also the concentrated solution at 34 (not all at once, 

 but gradually), yields the monohydrated salt, Na 2 SO 4 ,H 2 O. The hepta- 

 hydrated salt, Na 2 S0 4 ,7H 2 O, also splits up, even at low temperatures, 

 with the formation of this monohydrated salt, and therefore from 35 

 the solubility can be given only for the latter. For 100 parts of water 

 this is as follows : at 40 48-8, at 50 46-7, at 80 43'7, at 100 42-5 

 parts of the anhydrous salt. If the decahydrated salt be fused, and 

 the solution allowed to cool in the presence of the monohydrated 

 salt, then at 30 50-4 parts of anhydrous salt are retained in the solu- 

 tion, and at 20 52-8 parts. Hence, with respect to the anhydrous and 

 raonohydrated salts, the solubility is identical, and falls with increas- 

 ing temperature, whilst with respect to decahydrated salt, the solubility 

 rises with increasing temperature. So that if in contact with a solution 



solubility also decreases after a certain temperature is passed. Gypsum, CaSO4,2H 2 0, lime, 

 and many other compounds present such a phenomenon. An observation of Tilden's (1884) 

 is most instructive ; he showed that on raising the temperature (in closed vessels) above 

 140 the solubility of sodium sulphate again begins to increase. At 100 100 parts of 

 water dissolve about 48 parts of anhydrous salt, at 140 42 parts, at 160 43 parts, at 

 180 44 parts, at 230 46 parts. According to Etard (1892) the solubility of 80 parts of 

 Na-jSC^ in 100 of solution (or 43 per 100 of water) corresponds to 80, and above 240 the 

 solubility again falls, and very rapidly, so that at 320 the solution contains 12 per 100 of 

 solution (about 14 per 100 of water) and a further rise of temperature is followed by a 

 further deposition of the salt. It is evident that the phenomenon of saturation, deter- 

 mined by the presence of an excess of the dissolved substance, is very complex, and 

 therefore that for the theory of solutions considered as liquid indefinite chemical com- 

 pounds, many useful statements can hardly be given. 



7 Already referred to in Chapter I., Note 56. 



The example of sodium sulphate is historically very important for the theory of solu- 

 tions. Notwithstanding the number of investigations which have been made, it is still 

 insufficiently studied, especially from the point of the vapour tension of solutions and 

 crystallo-hydrates, so -that those processes cannot be applied to it which Guldberg, 

 Roozeboom, Van't Hoff, and others applied to solutions and crystallo-hydrates. It would 

 also be most important to investigate the influence of pressure on the various phenomena 

 corresponding with the combinations of water and sodium sulphate, because when crystal* 

 are separated for instance, of the decahydrated salt an increase of volume takes place, 

 as can be seen from the following data : the sp. gr. of the anhydrous salt is 2'66, that 

 of the decahydrated salt = T46, but the sp. gr. of solutions at 15/4 = 9,992 + 90'2j) + 0'35p 2 

 where p represents the percentage of anhydrous salt in the solution, and the sp. gr. of 

 water at 4 = 10,000. Hence for solutions containing 20 p.c. of anhydrous salt the sp. gr. 

 = ri936 ; therefore the volume of 100 grams of this solution = 83'8 c.c., and the volume of 

 anhydrous salt contained in it is equal to 20/2 '66, or =7'5 c.c., and the volume of water 

 = 80*1 c.c. Therefore, the solution, on decomposing into anhydrous salt and water, 

 increases in volume (from 83'8 to 87'6) ; but in the same way 83'8 c.c. of 20 p.c. solution 

 ore formed from (45'4/l'46 = ) Sl'l c.c. of the decahydrated salt, and 54'6 c.c. of water- 

 that is to say, that during the formation of a solution from 85'7 c.c., 83'8 c.c. are formed. 



