THE COLLOID-CHEMISTRY OF SOAPS 107 



2. Critical and Historical Remarks 



a. Introduction. Considered in the broad, these experiments 

 descriptive of the effects of different alkalies and of different 

 IK M it rai salts upon soap are as old as soap manufacture or chemical 

 industry itself. The precipitation of " insoluble " metallic soaps 

 by the addition of salts of the heavy metals to sodium or potas- 

 sium snaps is a familiar procedure in the manufacture of various 

 paint products; the addition of ammonium hydroxid to wash 

 waters has long been known to have a value in laundering proc- 

 esses not shown by more fixed " lyes "; and the " salting-out " 

 of soaps tlu-ough the addition of an excess of the alkali used in 

 making the soap or by the addition of ordinary sodium chlorid 

 is a century or more old. The soap chemists are also familiar 

 with the fact that in the salting-out process they often encounter 

 a " gumming " of their soap mixtures or find that these " go 

 strin Nevertheless, experimental details covering all these 



general subjects in more than partial fashion seem still to be 

 meagre, and the nature of the simplest findings seems not yet 

 to have emerged from the realm of harsh debate. 



If the facts and theoretical considerations covering the hydra- 

 tion and solvation properties of the pure soaps themselves as 

 previously outlined in these pages 1 are kept in mind, it l>eeomes 

 possible, we think, not only to draw together under a common 

 heading many of the empiric facts of chemical industry but to 

 find Jin explanation for them in decidedly simpler terms than 

 seem now to be in use. Before detailing tin views of other 

 workers in these fields we wish for the sake of clarity to divide 

 the experiments of this section into three groups. While the 

 phenomena discussed in any one of these commonly appear also 

 in a second, or even in Ji third. Mich divi>i>n \vill help to make 

 clear what it is that dominates behavior m each of th<> groups. 



The previous pages have sh<>\Mi that it is important to <tia- 

 tinguixh between the solubility of any soap in water and the solu- 

 bility of the water in that soap. Of immediate interest for our 

 .-it of the soaps of n m\-n fatty acid but 

 with different bases, ammonium soap is most soluble in water, 

 potassium next and then sodium. The soaps of th<- alkaline 

 earths are hardly soluble in water and those of the heavy metals 



' S.-.- prip- r,9 



