Super saturation of Salt-Solutions. 297 



Another experiment at 25° gave 



35-63 per cent. = 55-35 per 100. 

 Another pair at 30° 0. gave 



(1) 34-25,1 _ 51 . q4 Derl00 . 



(2) 34-13 ,J -^ 1J4 P ei wv> 



while another at 33° C. gave 



51-09 per 100. 



Before proceeding further I wish specially to lay stress on 

 the approximate nature of the above figures. They are all 

 necessarily high, for two reasons in particular. One is, that 

 it is impossible to filter the solution, for the instant it comes 

 in contact with the air crystallization commences ; thus, as it 

 is very dense and far from mobile, it is impossible to obtain 

 the solution perfectly clear. The second cause that raises the 

 percentage is, that a very considerable evolution of heat ac- 

 companies the solidification, and thus some of the water is 

 driven off, even when the dish is covered. In spite of this 

 the figures show clearly enough a decrease in solubility as the 

 temperature rises. 



As is w T ell known, the solubilitv-curve of Na 2 S0 4 ascends 

 rapidly to a point about 33° C. (Mulder 32°'75, Kopp 32°-93) 

 and then descends slowly to 100° C, after which it rises 

 again*. This change of solubility at 33° 0. is usually ascribed 

 to the dissociation of the decahydrate at that temperature ; 

 but I shall return to this. The most reliable data relating to 

 the solubility of Na 2 S0 4 are those given by Gray-Lussacf and 

 Mulder $. From the results of Gray-Lussac's experiments 

 Kopp§ constructed the following interpolation-formute. 



Below 30° C. The quantity dissolved in parts per 100 is 



(I.) Q = 5*02 + 0-30594^0-00041^ + 0-0009977^. 

 Above 40°, 



(II.) Q = 58-5-0-27783£ + 0-00069 * 2 + 0-0000049802 f 3 . 

 And the point of intersection of the two curves is given by 



-0-0009927* 3 + 0-00m 2 --0-58377£ + 53-48 = 0; 

 whence £ = 32-93. 



Now the agreement of the figures found by both Gay-Lussac 

 and Mulder with the values calculated by Kopp's formulas is 

 very complete, as is shown by the table, w r here Mulder's results 

 are compared with Kopp's figures. 



* Conf. Mulder, loc. cit. page 120. 

 t Ann. Chim. et Phys. [2] xi. p. 312. 

 * 1 Loc. cit. page 123. 



§ Ann. Chem. unci Pharm. 1840, xxxiv. p, 271. 



