508 PRINCIPLES OF CHEMISTRY 



at higher temperatures. 6 A concentrated solution at 34 has a composi- 

 tion nearly approaching to Na 2 SO 4 + 14H 2 O, and the decahydrated 

 salt, when mixed with 100 parts of water, contains 78*9 of the 

 anhydrous salt, from whence it is seen that the decahydrated salt 

 cannot fuse without decomposing, 7 similarly to hydrate of chlorine, 

 C1 2 ,8H 2 O, or sulphurous acid, SO 2 ,7H 2 O. 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 SO 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, and it is as follows for 

 100 parts of water : at 40 48-8, at 50 46-7, at 80 437, at 100 42'5 

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

 the solution be 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 

 monohydrated salts, the solubility is one and the same, and falls with 



6 In Chapter I. Note 24, we have already seen that with many other sulphates the 

 solubility also decreases after a certain temperature. Gypsum, CaSO4,2H 2 O, lime, and 

 many other compounds present such a phenomenon, not as yet, however, thoroughly 

 investigated. The observation of Tilden and Shenstone (1884) is most instructive, and 

 shows 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 

 43 parts of anhydrous salt, at 140 42 parts, at 160 43 parts, at 180 44 parts, at 280 

 46 parts. It is evident that the phenomenon of saturation, determined by the presence of 

 an excess of the dissolved substance, is very complex, and, therefore, for the theory of solu- 

 tions considered as liquid indefinite chemical compounds, many useful results can hardly 

 be given ; more especially as the physico-mechanical side of the transition of the solid 

 into the liquid condition (or the reverse) is up to the present time less clearly understood 

 in theory than the transition of a liquid into vapour. 



7 As has been referred to in Chapter I. Note 5(5. 



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

 tions. Notwithstanding the mass 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 crystals 

 are separated for instance, in 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 = 1-4(5, but the sp. gr. of solutions at 15/4 = 9992 + 90'2p + 0'35jp 2 

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

 water at 4 = 10000. Hence, for solutions containing 20 of anhydrous salt, the sp. gr. 

 - T193G ; therefore the volume of 100 grams of this solution = 88'8 c.c., and the volume of 

 anhydrous salt contained in it is equal to 20 2'(56, 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 83'8to87'6); but in the same way 83'8c.c. of 20p.c. solution are 

 formed from (45'4/l'4(> = ) 31'1 c.c. of the decahydrated salt, and 54'G c.c. of water that is 

 to say, that during the formation of a solution fromH5'7 c.c., 83'8c.c. are formed. 



