ON THE PHENOMENA AND THEORIES OF SOLUTION. 465 



as also do solutions of copper, nickel, and cobalt when dilute. But cad- 

 mium salts differ from all the rest in spreading to the edge of the blot. 



The water ring was widest when dilute solutions were used. 



Mr. Bayley's results have been confirmed and extended by J. U, 

 Lloyd (' Chem. News,' li. 51-54). 



Other porous substances, such as unglazed earthenware, behave in a 

 similar manner, and are even capable of depriving salts of water of 

 crystallisation, as observed by Potilitzin in the case of cobalt chloride 

 (' Ber.' xvii. 276). 



All these facts are undoubtedly connected, not only with the ascent 

 of liquids in tubes, but with the property which very finely divided, 

 though insoluble, powders generally possess of showing a rise of tempera- 

 ture ' when wetted, and of remaining suspended in a liquid in a state which 

 is sometimes referred to as pseudo-solution, until small quantities of 

 certain soluble matters are added ; that is to say, in every case there is 

 adhesion or surface attraction manifested between the solid and the hquid, 

 which is greater in proportion as the particles of the solid are smaller 

 and expose a greater surface ; and this adhesion is competent to separate 

 substances which are so closely and intimately united that everyone would 

 agree to say they were chemically combined. 



SuPERSATaRATION. 



The fascinating character of the phenomena has attracted a host of 

 experimenters, but no definite conclusion as to an explanation has been 

 generally accepted. 



Mr. Tomlinson, who a few years ago published many papers on the 

 subject, has given (' Proc. R. S.' xvi. 403) a history of the chief researches 

 up to his time. He has also arranged in five groups the salts he has 

 investigated according as they do or do not yield supersaturated solu- 

 tions, and according to the behaviour of those supersaturated solutions. 

 The following definition of supersaturation is given by Mr. Tomlinson 

 (loc. cit.) : ' When water at a high temperature is saturated with a salt, 

 and on being left to cool in a closed vessel retains in solution a larger 

 quantity of the salt than it could take up at the reduced temperature, the 

 solution is said to be supersaturated.' 



Such solations crystallise when brought into contact with a crystal of 

 the same salt, or of a compound truly isomorphous with it (J. M. 

 Thomson, ' Jour. Chem. Soc' 1879). 



Crystallisation, often in a modified form, is also in many cases brought 

 about when the solution is cooled to a low temperature or evaporated, 

 or when certain absorbent substances, such as paper or plaster, are 

 introduced into the liquid under certain conditions (Jeannel, ' Compt. 

 Rend.' Ixii. ; Grenfell, ' Proc. Bristol Nat. Soc' vol. ii. Part II. 130). 



It has been supposed by many chemists following the views expressed 

 in the earlier of the well-known researches of Lowel (' Ann. Chim.' [3] 

 xxix., xxxiii., xxxvii., xliii., xliv.) that supersaturation is due to the 



' Or, in the case of water, a fall in temperature, if below the temperature of 

 maximum density. See V. d. Mensbrugghe, Phil. Mag., [5] 2, p. 450, referring bo 

 Jungk's experiments. Since the above was written some interesting experiments 

 have been published by F. Meissner {Wiedemanti' s Annalen, 1886, p. 114), upon 

 the effect of moistening finely divided silica and other powders with water, benzene, 

 and amylic alcohol. In every case above 0° a very notable rise of temperature was 

 observed. 



1886. H H 



