of Solutions of Soda and Potash. 3G5 



Breaks in Densities. Hydrates isolated, and Breaks in freezing- 



point figure. 



49-6 ti c NaOH \ 41 ' 67 P' c> NaOH =NaOH, 3*H a O isolated. 



4- b p. c. JNaUM .... j (42 . 55 ^ =NaOH, 3H 2 0) 



38-83 „ =NaOH,3-5H 2 „ 



35-71 „ = NaOH,4H„0 „ 



30-77 „ = NaOH,5H 9 ~0* ,, 



24-10 „ =NaOH,7H 2 „ 



19-80 „ = NaOH,9H 2 „? 



12-3 „ =NaOH, 1611,0 break. 



4 „ =NaOH,53H20 „ 



38-77 



35-0 



29-4 



24-05 



20-0 



12-1 



3-4 



The above list gives the positions of the eight breaks 

 established by the densities, and of these, it will be seen, six 

 certainly agree with six of the hydrates which have been 

 isolated in the crystalline condition; and, although the remain- 

 ing two occur in the region of weak solutions where no 

 hydrates were actually isolated, still the freezing-points in 

 this region show two well-marked breaks (and these were the 

 only two shown here), and both of these agree well with 

 those in the density-figure. 



Such a thorough agreement between the breaks and the 

 hydrates isolated must, I think, settle definitely that these 

 breaks are really due to the hydrates present in solution. 



The first isolated hydrate mentioned in the above list is a 

 complex one,NaOH,3^H 2 0, or, as I pointed out at the time (loc. 

 cit. p. 894) , probably a molecular compound of 8(NaOH,3H 2 0) 

 with NaOH, 4H 2 0. But the existence of such a complex 

 necessitates also the existence of the hydrate NaOH, 3H 2 0, 

 though it itself was not actually isolated, and it is this tri- 

 hydrate, and not the complex of it with the tetrahydrate, 

 which seems to be shown by the densities. The composition 

 of the trihydrate and its complex differs but little, and the 

 density-determinations could not possibly have shown a break 

 corresponding to each. It is probably due to the existence of 

 the undiscovered break here that the densities appear to be 

 slightly discontinuous. 



A similar explanation may hold good in the case of the 

 apparent discontinuity at the tetrahydrate : two hydrates of 

 about this composition were, as a matter of fact, isolated, and, 

 although the analytical numbers pointed to their being 



* The composition of this hydrate, as given by experiment, was 

 NaOH, 5 T 2 T H 2 0, containing 30 - 01 per cent. soda. This value agrees 

 better with the position of the density-break than does the peutahydrate 

 entered in the table. The freezing-points, however, were scarcely 

 sufficient to settle whether it was a pentahydrate or the more complex 

 hydrate mentioned. 



