<'N WATKR AND ITS COMPOUNDS 95 



<! ystallisation to the amount of Na 2 S0 4 ,10H 2 O that is, 180 parts of 

 water for 142 parts of anhydrous salt. The supersaturated solution 

 may be moved about or shaken inside the vessel holding it, and no 

 i-ry>tallisatioii will take place; the salt remains in the solution in as 

 laruv an amount as at. a higher temperature. If the vessel holding 

 the supersaturated solution be opened and crystals of Glauber's salt be 

 thrown in, crystallisation suddenly takes place. 53 A considerable rise 

 in temperature is noticed during this rapid separation of crystals, which 

 is explained by the salt, previously in a liquid state, passing into a solid 

 state, by which, as is known, latent heat is evolved. This somewhat 

 resembles the fact that water may be cooled below (even to 10) if 

 it be left at rest, under certain circumstances, and evolves heat in 

 suddenly crystallising. Although from this point of view there is a 

 resemblance, yet in reality the phenomenon of supersaturated solutions 

 is much more complicated. Thus, on cooling, a saturated solution of 

 Glauber's salt deposits crystals containing Na 2 SO 4 ,7H 2 O, 56 or 126 parts 



65 Inasmuch as air, as has been shown by direct experiment, contains, although in 

 very small quantities, minute crystals of salts, and among them of sodium sulphate, air 

 can bring about the crystallisation of a saturated solution of sodium sulphate in an open 

 vessel, but it has no effect on saturated solutions of certain other saTts ; for example, lead 

 acetate. According to the observations of De Boisbaudran, Gernez, and others, isomor- 

 phous salts (analogous in composition) are capable of evoking crystallisation. Thus, a 

 supersaturated solution of nickel sulphate crystallises by contact with crystals of sul- 

 phates of other metals analogous to it, such as those of magnesium, cobalt, copper, and 

 manganese. The crystallisation of a supersaturated solution, brought about by the con- 

 tact of a minute crystal, starts from it in rays with a definite velocity, and it is evident 

 that the crystals as they form propagate the crystallisation in definite directions. This 

 phenomenon recalls the evolution of organisms from germs. An attraction of similar 

 molecules ensues, and they dispose themselves in definite similar forms. 



56 In these days a view is very generally accepted, which regards supersaturated 

 solutions as homogeneous systems, which pass into heterogeneous systems (composed of 

 a liquid and a solid substance), in all respects exactly resembling the passage of water 

 cooled below its freezing point into ice and water, or the passage of crystals of rhombic 

 sulphur into monoclinic crystals, and of the monoclinic crystals into rhombic. Although 

 many phenomena of supersaturation are thus clearly understood, yet the spontaneous for- 

 mation of the unstable hepta-hydrated salt (with 7H 2 O), in the place of the more stable 

 deca-hydrated salt (with mol. 10H 2 O), indicates a property of a saturated solution of sodium 

 sulphate which obliges one to admit that it has a different structure form an ordinary 

 solution. Stcherbacheff affirms, on the basis of his researches, that a solution of the 

 deca-hydrated salt gives, on evaporation, without the aid of heat, the deca-hydrated salt, 

 whilst after heating above 33 it forms a supersaturated solution and the hepta-hydrated 

 salt, which gives reason for thinking that the state of salts in supersaturated solutions 

 is different from that in ordinary solutions. But in order that this view should be 

 accepted, some signs must be discovered distinguishing solutions (which are, according to 

 this view, isomeric) containing the hepta-hydrated salt from those containing the deca- 

 liydrated salt, and all efforts made in this direction (the study of the properties of the 

 solutions) have given negative results. Further, according to this view, one would expect 

 that all supersaturated solutions would contain particular forms of crystallohydrates, 

 ami, although this is possible, yet up to now nothing of the kind has been observed, 



