FORMATION OF OIL-FOAM 19 



and quickly dissolve in it, and by their action on the oil 

 call forth, sometimes in one place, sometimes in another, 

 local extension-currents. At this period, and usually also at 

 the moment when the cover-slip with the drop of paste is laid 

 on the drop of water, a more or less considerable number of 

 small and minute oil droplets separate off from the drop of 

 paste, for which reason the latter generally becomes sur- 

 rounded by a zone of such little droplets. If the oil is well 

 suited to the production of foam, these little droplets become, 

 so to speak, instantaneously converted into foam-drops, so 

 that it can be thus determined with some certainty if 

 the oil in question is in the smallest degree not quite 

 suitable. In the drops of paste there gradually appears an 

 increasing number of larger or smaller droplets of fluid, as a 

 result of which it becomes more and more opaque. From 

 time to time eruptions of such drops of fluid into the sur- 

 rounding water take place on its free surface, which are 

 naturally accompanied in their turn by superficial extension- 

 currents. These extension-currents doubtless also promote 

 the conversion of the drop into froth, since, as will be 

 shown later, violent extension -currents frequently bring 

 about absorption of the surrounding fluid in the form of 

 minute drops. In a relatively short time the drop of paste 

 becomes quite opaque and milky white in transmitted light. 

 With the extinction of the currents from the interior and of 

 the superficial extension-currents, the drop, which formerly 

 had more or less irregular and variable contours, rounds itself 

 off, for the most part, completely, and then finally remains 

 perfectly quiescent. This rounding off of the drop usually 

 takes place in a relatively short time — about one-half to one 

 hour. After about twenty-four hours the process is entirely 

 ended, and the drop is suitable for further investigation. 



Naturally, with this conversion of the oil-drop into foam, 

 its volume increases considerably, upon which I shall make 

 more precise statements below in the proper place. As a 

 rule, during the process more or fewer bubbles of gas, 

 i.e. CO^, appear in the drop, which are expelled from 

 the KgCOg ' by the free oleic acid ; later they gradually 

 vanish again. I do not, however, think that the forma- 



