STREAMING OF FOAM-DROPS EXPLAINED 75 



this diminution of tension can only be soap, which spreads 

 itself out in solution upon the surface of the drop. Accord- 

 ing to the statements given above as to the origin and 

 structure of the foams, the fluid which fills their alveoli is a 

 watery (or after clearing them, a glycerine - containing) 

 solution of soap. Both by diffusion and by bursting of 

 superficial alveoli, the soap solution in the contents of the 

 alveoli comes to the surface and produces extension- 

 currents. I should conclude that bursting of alveoli does 

 actually come into play from the fact that when the foams 

 are brought into water, local eruptions of some strength 

 frequently break out from the interior, and are accompanied 

 by strong superficial extension-currents. That such erup- 

 tions depend upon occasional bursting of single larger 

 vacuoles is beyond a doubt. 



When we turn to details, an explanation ought first to 

 be given for the peculiar circulatory currents, which the 

 drops of foam, when free from pressure, usually show after 

 being transferred to glycerine. But first of all, a word as to 

 the influence of glycerine may not be out of place. As has 

 been already described above, the drops move about also in 

 water, whence it follows that glycerine does not produce the 

 movements. On the other hand, I quite believe that it 

 favours them to a certain degree, since, in particular, it 

 hinders, or at least considerably diminishes, the adhesion 

 between the drop and the glass. I am not inclined to 

 ascribe to the addition of glycerine any other influence upon 

 the movement. 



The peculiar circulatory streaming which the drops of 

 foam show, as a rule, after transference to glycerine may be 

 explained, upon the basis of the results now obtained, in the 

 following manner. If the cause of the phenomena of stream- 

 ing is to be sought in the exudation of soap solution at 

 the surface of the drop, it being a matter of indifference 

 whether this takes place by diffusion or by bursting of 

 alveoli, it follows that if this exudation at the surface takes 

 place at first fairly evenly, there must be soap solution of 

 greater concentration formed in the narrow interstices which 

 remain between the surface of the drop and the cover glass 



