210 kEPORl'fcl OA THt; StAtfe Ol<' SClENCtl. 



not merely llionomolecularj but almost wholly ionised.' The extloC 

 physical sigiiiticance of p therefore remains to be determined, but where 



the case is one of surface-action is obviously an arbitrary expression, 



which is proportional for any one adsorbent in a uniform state of division, 



but gives no indication of the absolute surface involved. Even if the 



surface is molecular, as it probably is in jellies, we have little knowledge 



of the complexity of the molecules, it can hardly be doubted that 



adsorption is dependent on the same forces as solution, as it is obviously 



selective and influenced by the chemical relations of the adsorbed and 



adsorbing substances, and in some cases is a preliminary to actual chemical 



combination. 



JPreundlich'^ has suggested a somewhat diH'erent em^airical formula from 



that given above for expressing adsori^tion, the so-called X formula, in 



which A. for a given original concentration of the adsorbed solution is 



constant and independent, of the quantity of adsorbent, and takes the place 



ot /J in the simple equation expressing Henry's law. If, however, 



the concentration of the solution (i.e., the relation of the dissolved sub- 



/'a\ _- 

 stance a to the volume of the solution v) is varied, /\=a I - ) ", where 



a and n are constants dependent only on temperature and the nature of 

 the dissolved substance. This formula has been severely criticised by 

 Bain,-* who shows that in some cases it leads to quite anomalous results. 

 Freundlich adopts the view of Lagergreen,^ that adsorption is due to 

 surface-tension, and points out that a dissolved body which lowers the 

 surface-tension between solution and solid must become concentrated on 

 that surface. That this is an actual factor in adsorption admits of no 

 doubt ; but our knowledge is not jet sufficient to enable us to calculate 

 its magnitude ; and it would seem to imply that adsorption was inde- 

 pendent of any direct relation between the adsorbed and adsorbing body, 

 which does not appear to be the case. It may be, however, that chemical 

 relations between the solid and the dissolved substance influence the 

 tensions of the solid-liquid surface, and so aflect the results. 



Inorganic sols are frequently coagulated by freezing or evaporation, 

 and this is often really due to the traces of electrolytes always present in 

 llie sols, which become concentrated in the remaining liquid till they 

 reach the minimum necessary for tlocculation. 



Colloid sols are not only flocculated by ions but by other sols of oppo- 

 site charge, the two colloids precipitating together in a way which closely 

 simulates an ionic reaction ; and many precipitations usually considered 

 chemical, such as those of tannin and gelatiue, tannin and bisic colours, 

 and basic with acid colours are of this character. If the two sols are 

 present in exactly the right proportions the ptecipitatijn is complete, a^d 

 the precipitate ot almost constant composition ; but if one be in excess 

 neither is completely precipitated. Probably in this Cdse, as in tloccu- 

 lation by electrolytes, there is a miuimum coucentration below which pre- 

 cipitation does not take place. 



The cause of this minimum concentration being needed is not clear, 



' Cp. Walker and Appleyard, Traits. C'h. Sue, 189(3, 69, 1334. 



* ' Ueber die Absorptioo ia Losungeu,' UabUitatiousschri/t, Leipzig, 1906. 

 » Trans. Lh. Sor., 1907, 91, 1683. 



* mhany t. Scumka Vvt. Ak. MauM., 1899, 24, II., Nr. 4, 49; /i. 2>ht/s. CL, IJOO 

 82, 174, 175. 



