LIST OF ILLUSTEATIONS 



PRINTED IN THE TEXT 



1. Formation of foam in an oil-drop placed in water. The darker peri- 



pheral zone represents the portion which has become frothy, while 

 the centre of the drop is occupied by clear homogeneous oil. NaC^, 

 fragments of common salt ; u, vacuoles . . . .12 



2. Diagram of the union of three lamellae of a foam in one edge, as seen 



in optical section . . . . . . .27 



3. Diagram representing an optical section of the superficial region of a 



foam. The arrows a, t, c, represent the tensions of three lamella 



at the surface . . . . . . .33 



4. Diagrammatic optical section of the greatly attenuated margin of a 



drop of oil-foam ....... 37 



5. Diagram showing the course of the currents in a drop of oU-foam 



placed in dilute glycerine, o, the upper, a, the lower side of the 

 drop ; 0, slide ; D, cover glass . . . . .48 



6. Outline sketch showing one of the forms assumed by a large drop of 



oil-foam in active movement. The pairs of dotted arrows repre- 

 sent centres of extension-currents . . . . .51 



7. Outliue sketches showing the forms assumed by a drop of oil-foam, 



when warmed and moving actively, during a period of ten minutes. 

 Arrows to denote extension-currents . . . .53 



8. Outline sketches of a drop of oU-foam under the influence of the 



electric currents ; taken at intervals of five minutes, to show the 

 changes of form and movement consequent upon change of the 

 poles. The signs -I- and - denote the positive and negative poles ; 

 the arrows denote the extension-currents . . . . 58« 



9. Diagi-am showing the system of extension-currents produced in a 



drop of oil placed in water under the cover glass, when approached 

 on one side by soap solution. The arrows denote the currents. 

 m, the axial quiescent streak ; x, the posterior quiescent region . 62 

 10. Diagi-am showing extension-currents in a drop of paraffin oil under 

 similar circumstances, and also the currents in the surrounding' 

 water . . • ■ • • • . - 64 



