402 Dr. W. A. Tilden. Influence of Temperature [June 18, 



Crystallised sodinm sulphate melts at about 34°, and at this tem- 

 perature the salt begins to show signs of dissociation by depositing 

 the anhydrous compound. According to Wiedemann,* indeed, indi- 

 cations of change are observable a,t a few degrees lower. He finds 

 by observation of the volume of the crystallised salt at different tem- 

 peratures, that whereas slight expansion occurs from 17° to about 30°, 

 contraction then sets in, at first slowly, then, at 33° to 34°, very rapidly, 

 till the salt melts. The melted salt expands regularly in proportion 

 as the temperature is raised to near 100°. Nicolf has made experi- 

 ments upon the expansion of solutions of the same salt, and with 

 similar results. 



The solubility of sodium sulphate increases rapidly from ordinary 

 temperatures up to 34°, when it attains a maximum. From thi 

 upwards the solubility again diminishes very rapidly to between it 

 and 50°, and then less rapidly, till at 100° to 160° it becomes nearly 

 stationary. At temperatures higher than 160° the solubility slowly 

 increases again. J 



In 1874 it was observed by De Coppet§ that heat is developed by 

 contact of water with anhydrous sodium sulphate at temperatures 

 considerably above 34°, at which point the ordinary hydrate is broken 

 up. This circumstance was attributed by Thomsen to the formation 

 of a monohydrate, Na 2 S0 4 .H 2 0, and he ascribed this composition to 

 the crystals which are deposited on heating a concentrated solution of 

 sodium sulphate, and which had always been believed to be anhy- 

 drous. Thomsen's assumption has, however, been shown to be ground- 

 less, || and therefore the fact noticed by De Coppet remains without 

 adequate explanation. 



Below the temperature of 34° it seems probable that the sodium 

 sulphate dissolved in water is in the state of hydrate, Na 2 SO 4 10H 2 O. 

 In what condition is it at temperatures higher than 34° ? In other 

 words, does the solution contain the usual hydrate or the anhydrous 

 salt, or both ? 



This is the question which I have endeavoured to attack by com- 

 paring the thermal changes attending the act of solution of the anhy- 

 drous salt in water at temperatures above this critical point with the 

 corresponding change at temperatures below. Some interesting re- 

 sults have been obtained, but they do not supply a final answer to this 

 question. My few earlier experiments, communicated in a short rote 

 to the British Association at Montreal, gave erroneous results, partly 



* " Pogg. Ann.," 1882, 561. 



f " Ber. Deut. Chem. Ges.," xv, 1931 b. 



X Tilden and Shenstone, " Phil. Trans.," 1884, I, 23. 



§ " Comp. Kend.," 79, 167. 



|| De Coppet, "Ber. Deut. Chem. Ges.," xii, 248, and Pickering, "Jour. Chem. 

 Soc," 1884, 689. 



