STUDY OF MOLECULAR ADHESION. 427 



We found that the flocculation of the complex, barium sulphate 

 plus gum, by a salt, is reversible, that is to say, on the removal of 

 the saline solution and suspension in distilled water, the dissemin- 

 ated condition comes back. In the same way the complex, barium 

 sulphate plus citrate, flocculated by a salt, and after the removal of 

 this salt suspended in distilled water, becomes disseminated again. 

 There is then a distinct analogy between the complexes of barium 

 sulphate plus colloid and the complex, barium sulphate plus citrate, 

 in respect to flocculation by electrolytes. A reversibility of the 

 flocculation of the citrate combination by means of a salt also 

 occurs at least within the limits of saline concentration which 

 we have employed on agglutinating the complexes by a 

 soluble calcium salt in place of the sodium salt; the complex after 

 the excess of calcium solution is removed when suspended in distilled 

 water recovers its normal appearance. We have also noted that 

 if barium sulphate, NaCl, and sodium citrate are mixed together all 

 at once, that the complex of barium sulphate plus citrate still forms; 

 it is quite as readily flocculated by the sodium chloride present, 

 but when washed and resuspended in distilled water it acts as if it 

 had been formed without the presence of sodium chloride. 



*** 



To sum up, sodium citrate forms a complex with barium sulphate 

 as with other insoluble substances which causes a fragmentation of 

 the granules of the suspension into extremely small suspended par- 

 ticles; this results in the formation of a much more delicate and 

 stable suspension of the sulphate. The same thing is true with clay. 

 Arthus has, indeed, noted that the flocculation of clay by a sodium 

 salt requires much more salt when citrate is present than when it 

 is absent. The inhibition which citrate causes on the flocculation 

 of a suspended substance in water by means of a salt may be made 

 use of in detecting the formation of a complex between such a 

 suspension and citrate. We may use this fact to advantage in deal- 

 ing with a material which usually gives so fine a suspension in water 

 that the addition of citrate produces no visible modification. 

 Such is the case with calcium fluoride (fine suspensions of calcium 

 fluoride in water which are almost colloidal may be obtained), 

 and particularly with mastic. Both these substances, which are 



