o.\ WATEB AND ITS COMPOUNDS 71 



of ;i curve that i>, having made a determination of the solubility for 

 several temperature tin- solubility at intermediary temperatures may 

 be determined from the sinuosity and form of the curve so formed ; in 

 this way the empirical law of solubility may be followed.' 2 " 1 The results of 

 research have shown that the solubility of certain salts as, for example, 

 (in 11 moil table salt varies comparatively little with the temperature ; 

 whilst for other substances the solubility increases by equal amounts for 

 equal increments of temperature. So, for example, for the saturation of 



-' (lay-Lu-^ac \vas the first to have recourse to such a graphic method of expressing 

 solubility, and lie considered, in accordance with the general opinion, that by joining up 

 the summits of the ordinates in one harmonious curve it is possible to express the entire 

 change of solubility with the temperature. Now, there are many reasons for doubting 

 the accuracy of such an admission, for there undoubtedly are critical points in curves of 

 solubility (for example, of sodium sulphate, as shown further on), and it may be that 

 definite compounds of dissolved substances with water, in decomposing within known 

 limits of temperature, give critical points more often than would be imagined; it may 

 even be, indeed, that instead of a continuous curve, solubility should be expressed if 

 not always, then not unfrequently by straight or broken lines. According to Ditte, the 

 solubility of sodium nitrate, NaXO,-, is expressed by the following figures per 100 parts of 

 water : 



1 10 15 21 29 36 51 68 



()C>-7 71-0 7<i'o 80-6 85'7 92".> 91)'4 13'6 12;V1 



According to my opinion (iHHlj, these data should be expressed with exactitude by a 

 straight line. (\7',~> -f O'STf, which entirely agrees with the results of experiment. Accord- 

 ing to this the figure expressing the solubility of the salt at exactly coincides with 

 the composition of a definite chemical compound NaXO5,7H 2 O. The experiments 

 made by Ditte showed that all saturated solutions between and 15'7 have such a 

 composition, and that at the latter temperature the solution completely solidifies into one 

 homogeneous whole. Ditte shows, in the first place, that the solubility of sodium nitrate 

 is expressed by a broken straight line, and, in the second place, confirms the idea, 

 which I had already traced, that in solutions we have definite chemical compounds in a 

 state of dissociation. In recent times (IHHH) Etard discovered a similar phenomenon in 

 many of the sulphates. Brandes, in 1830, shows a diminution in solubility below 100 

 for manganese sulphate. The percentage by weight (i.e., per 100 parts of the solution, and 

 not of wateri of saturation for ferrous sulphate, FeSO 4 , from 2 to + 65 = 13'5 + 0'3784f 

 that is, the solubility of the salt increases. The solubility remains constant from 65 to 

 98 (according to Brandes the solubility then increases ; this divergence of opinion 

 requires proof), and from 98 to 150 it falls as = 104'35- 0'6685. Hence, at about 

 + 156 the solubility should =0, and this has been confirmed by experiment. I observe, 

 on my side, that Etard's formula gives 38'1 p.c. of salt at (55 and 38"8p.c. at 92, and this 

 maximum amount of salt in the solution very nearly corresponds with the composition 

 FeSO 4 ,14H 2 O, which requires 37'6 p.c. Thus, in this case, as in that of sodium nitrate, 

 the formation of a definite solution may be presupposed. From what has been said, it is 

 evident that the data concerning solubility require a new method of investigation, which, 

 in the first place, should have in view the entire scale of solubility from the formation 

 of completely solidified solutions (cryohydrates, which we shall speak of presently) to the 

 separation of salts from their solutions, if this is accomplished at a higher temperature 

 (for manganese and cadmium sulphates there is an entire separation, according to Etard), 

 or to the formation of a constant solubility (forpotassium sulphate the solubility, accord- 

 ing to Etard, remains constant from 163 to 220 and equals 24'9 p.c.) ; and, in the second 

 place, should endeavour to apply the conception of definite compounds existing in solu- 

 tions to constant and critical solutions, corresponding with a maximum of solubility or 

 of its limits. From these aspects solution should present a new and particular inter. 



