STUDY OF MOLECULAR ADHESION 429 



The opacity produced by sodium chloride may be due to a double 

 decomposition: the sodium citrate could not give the insoluble 

 substance, aluminium, with sodium citrate if it reacted chemically 

 with sodium chloride, but could give only soluble substances. We 

 must then be dealing with a flocculation of very small particles 

 in limpid colloidal solution by sodium citrate; these particles must 

 belong to the complex, aluminium plus citrate. This complex, then, 

 would be analogous to the complex of barium sulphate plus citrate, 

 but instead of being a fine suspension, as this complex, it would 

 appear in water as an apparently colloidal limpid solution. Such 

 a mechanism probably accounts for the inhibition to precipitation 

 of aluminium salts by bases which citrate gives. 



Notwithstanding, the reasons which seem to favor this opinion, 

 we give this interpretation as an hypothesis only. The subject 

 should be studied more carefully from a purely chemical stand- 

 point. 



We should like, however, to mention one more fact which is re- 

 lated to the phenomenon that we have just described. We know 

 that the salts of aluminium, for example the sulphate, are coagu- 

 lants and agglutinants. Alum flocks certain dyes, such as fuchsin, 

 with ease. We have found that such a flocculation is completely 

 inhibited by sodium citrate. This effect cannot be due to a pre- 

 cipitation of the agglutinating element from the solution, for we 

 find that these salts show no precipitation after several days. It 

 may be that the obstacle to the flocculation by alum produced by 

 citrate is due to the dissemination of alum hydrate, which would 

 remove its precipitating properties. This, to be sure, is only a 

 hypothesis, the value of which must be determined only by more 

 extensive experiments. 



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The inhibiting power of sodium citrate on the flocculation of 

 certain substances by salts (Arthus) is better understood when we 

 consider its disseminating effect on substances or elements which 

 sediment spontaneously. In the same way as the suspension of 

 insoluble substances by stable colloids is to be considered as the 

 result of the formation of a complex between the powder and the 

 colloid, so the disseminating action of sodium citrate on certain 



