460 



SCIENCE 



[N. S. Vol. XL. No. 1031 



addition, the force has zero value at a 

 second distance lying between the first zero 

 and the molecular range, with accompany- 

 ing alternations in the sign of the force. 

 Thus, starting from zero distance apart of 

 the particles, the sign of the force is nega- 

 tive or repulsive; then, as the distance 

 apart is supposed to increase, the force of 

 repulsion diminishes, and after passing 

 through zero value becomes positive or at- 

 tractive; nest, as the distance is increased 

 the force diminishes again, and after 

 passing through a second zero becomes 

 negative for a second time; finally, the 

 force on passing through a third zero be- 

 comes positive, and is then in the stage 

 dealt with in capillary aud other ques- 

 tions. 



As an instance, of where these alterna- 

 tions of sign seem to be manifest, may be 

 mentioned the ease of certain crystals 

 when split along cleavage planes. The 

 split often runs along further than the 

 position of the splitting instrument or in- 

 serted wedge seems to warrant. This 

 would occur if the particles on either side 

 of the cleavage plane were situated at the 

 distance apart where the force between 

 them was in the first attractive condition, 

 for then on increasing the distance be- 

 tween the particles by means of the wedge 

 the force changes sign and becomes repul- 

 sive, thus helping the splitting to be prop- 

 agated further out. 



Assuming that a repulsive force can 

 supervene between the particles in the ad- 

 sorption layer, through the particles be- 

 coming so crowded in places as to reduce 

 their mutual distances to the stage when re- 

 pulsion sets in, we might expect that an 

 instability would be set up. 



As already stated, a rise in temperature 

 reduces in general the amount adsorbed, 

 but below the critical point the nitrates 

 and sulphates are exceptional, for rise in 



temperature here increases the amount ad- 

 sorbed from a given solution. This ob- 

 viously necessitates that the isothermals 

 cross one another at the critical point in 

 an adsorption-concentration diagram. This 

 may perhaps account for some observers 

 finding that adsorption did not change 

 with temperature. We have another ex- 

 ception to the simple laws of adsorption in 

 the case of the alkali chlorides ; this excep- 

 tion occurs under certain conditions of 

 temperature and strength of solution. The 

 normal condensation into the surface layer 

 is reversed and the salt is repelled into the 

 general solution instead of being attracted 

 by the surface. In other words, it is the 

 turn of the other constituent of the solu- 

 tion, namely, the water, to be adsorbed. 



It is a very well known experiment in 

 adsorption to run a solution such as that 

 of permanganate of potash through a filter 

 of sand, or, better, one of precipitated 

 silica, so as to provide a very large sur- 

 face. The first of the solution to come 

 through the filter has practically lost all its 

 salt, owing to having been adsorbed by the 

 surface of the sand. 



I was interested in finding a few months 

 ago that Defoe, the author of "Robinson 

 Crusoe," in one of his other books, depicts 

 a party of African travelers as being saved 

 from thirst in a place where the water was 

 charged with alkali by filtering the water 

 through bags of sand. Whether this is a 

 practical thing or not is doubtful, or even 

 if it has ever been tried ; for it is only the 

 first part of the liquid to come through the 

 filter which is purified, and very soon the 

 surface has taken up all the salt it can ad- 

 sorb, and after that, of course, the solution 

 comes through intact. It is interesting, 

 however, to know that so long ago as De- 

 foe's time the phenomenon of adsorption 

 from salt solutions had been observed. It 

 is not so well known that in the case of 



