504: Johnston, M'erwi?i; and Williamson — 



of Lashchenko,* according to whom there is an evolution of 

 27 cal. per gram; but this estimate is based upon a misappre- 

 hension of his results. For his method could yield only the 

 total heat evolved when either aragonite or calcite cooled from 

 465° to 20°, and the difference therefore would represent 

 merely the difference in heat capacity over this temperature 

 range ; but his observed difference is far greater than that 

 deduced from the most trustworthy series of experiments'!" 

 (viz. 1*8 cal. per gram) and his high value is due probably to 

 some dissociation of the aragonite with the production of free 

 lime. 



Altogether then this heat of transformation is probably 

 small, but its sign is unknown. Incidentally it may be remarked 

 that, on the basis of the values of specific heat cited above, the 

 heat of transformation per gram will be at 470° about 1*8 cal. 

 algebraically less than it is at 20° ; so that, if the heat effect is 

 small and positive at 20°, it may at some higher temperature 

 become zero and reverse its sign. 



The Precipitation of Unstable Forms. 



In taking up this important question let us consider first the 

 course of events when solutions of, say, sodium sulphate and 

 barium chloride are gradually added to a large volume of 

 water, ultimately producing barium sulphate, a substance 

 which, so far as we know, appears only in a single crystalline 

 form. The concentration of the barium ion [Ba ++ J and of 

 sulphate ion [SO 4] increase, and hence their product [Ba ++ ] 

 [SOj] gradually increases, but as soon as this product has 

 attained a value 1£- — viz., the characteristic solubility-product 

 constant appropriate to the temperature — precipitation begins 

 provided that nuclei are present ; but if nuclei are not present, 

 some degree of supersaturation;}: will be attained before pre- 

 cipitation begins. Now the size of the precipitated particles 

 depends upon the degree of supersaturation at the moment of 

 precipitation, the particles tending to be smaller (and more 

 numerous) the greater the supersaturation, § and consequently 



* Lashchenko, J. Russ. Phys. Chem. Soc, xliii, 793, 1911. We are 

 indebted to Dr. E. W. Posnjak, who translated portions of this article for us. 



f Lindner, Sitzber. physik. medizin. Ges. , Erlangen, xxxiv, 217, 1902. 



% The degree of supersaturation required to induce the separation of nuclei 

 of any particular substance depends upon conditions, e. g. , upon the mode 

 of stirring and the other substances present in the liquid. (See L. C. de 

 Coppet, Ann. chim. phys., x, 457, 1907; S. W. Young, J. Am. Chem. Soc, 

 xxxiii, 148, 1911 ; S. W. Young and R. J. Cross, ibid., 1375, 1911. There is 

 therefore no definite curve of supersaturation, as was postulated by Miers and 

 Isaac (Proc. Roy. Soc, London, A, lxxix, 322, 1907; F. Isaac, J. Chem. Soc, 

 xciii, 384, 1908 ; Miers and Isaac, ibid., xciii, 927, 1908 ; B. M. Jones, ibid., 

 xciii, 1739, 1908.) 



§Cf. Table I, experiment No. 33, where the momentary supersaturation 

 was so great that a jelly resulted. 



