OCTOBEB 2, 1914] 



SCIENCE 



459 



As far as investigations already made go, 

 the laws of adsorption appear to be very 

 complicated, and no doubt many of the 

 conflicting experimental results wbieh have 

 been obtained, are in part due to this, 

 workers under somewhat different condi- 

 tions obtaining apparently contradictory 

 effects. 



On the whole, however, it may be said 

 that the amount adsorbed increases with 

 the strength of solution according to a 

 simple power law, and diminishes with rise 

 of temperature ; but there are many excep- 

 tions to these simple rules. For instance, 

 in the case of certain sulphates and ni- 

 trates the amount adsorbed by the surface 

 of, say, precipitated silica, only increases 

 up to a certain critical point as the 

 strength of the solution is increased. 

 Then further increase in the strength of 

 the solution causes the surface to give up 

 some of the salt it has already adsorbed or 

 the amount adsorbed is actually less now 

 than that adsorbed from weaker solutions. 

 Beyond this stage for still greater concen- 

 trations of the solutions the amount ad- 

 sorbed goes on increasing as before the 

 critical point was reached. 



There is some reason for thinking that 

 there are two modes in which the salt is 

 taken up or adsorbed by the solid surface. 

 The first of them results from a simple 

 strengthening of the solution in the sur- 

 face layers; the second, which takes place 

 with rather stronger concentrations, is a 

 deposition in what is apparently analogous 

 to the solid form. It would seem that the 

 first reaches out from the solid surface to 

 about 10"^ em.- — which is the order of the 

 range of attraction of the particles of the 

 solid substance. 



The cause of the diminution in the ad- 

 sorption layer at a certain critical value of 

 the concentration is difficult to understand. 

 Something analogous has been observed by 



Lord Eayleigh in the thickness of layers of 

 oil floating on the surface of water. As oil 

 is supplied the thickness goes on increas- 

 ing up to a certain point, beyond this, on 

 further addition of oil, the layer thins 

 itself at some places and becomes much 

 thicker at others, intermediate thicknesses 

 to these being apparently unstable and un- 

 able to exist. As helping towards an ex- 

 planation of the diminution in the adsorp- 

 tion layer we may suppose that as the 

 strength of the solution is increased from 

 zero, the adsorption is at first merely an 

 increased density of the solution in the 

 surface layer. For some reason, after 

 this has reached a certain limit, further 

 addition of salt to the solution renders 

 this mode of composition of the surface 

 layers unstable, and there is a breaking up 

 of the arrangement of the layer with a dim- 

 inution in its amount. "We may now sup- 

 pose the second mode of deposition to be- 

 gin to show its effect with a recovery in the 

 amount of the surface layers and a 

 further building up of the adsorption de- 

 posits. 



On account of passing through this point 

 of instability the process is irreversible, so 

 that the application of thermo-dynamics to 

 the phenomenon of adsorption is necessar- 

 ily greatly restricted in its usefulness. 



A possible cause of the instability in the 

 adsorption layer which occurs at the crit- 

 ical point may be looked for in the alter- 

 nations in the sign of the mutual forces be- 

 tween attracting particles of the kind sug- 

 gested by Lord Kelvin and others. Within 

 a certain distance apart — the molecular 

 range — the particles of matter mutually 

 attract one another, while at very close 

 distances they obviously must repel, for 

 two particles refuse to occupy the same 

 space. At some intermediate distances the 

 force must pass through zero value. It has 

 for various reasons been thought that, in 



