FORMATION OF FROTH IN OIL 9 



above-mentioned olive oil by means of either cane sugar or 

 common salt into a very fine froth, while the attempts with 

 potassium nitrate did not yield favourable results, and hence 

 were not further continued. The behaviour of the oil-drops 

 when brought into water is, as far as it was followed out, 

 somewhat as follows. Microscopic observation shows, in the 

 first place, that the pulverisation of the substance mixed in 

 with the oil is, in spite of all care, comparatively coarse, 

 and that, besides particles of the finest size, a considerable 

 number of coarse fragments are present in addition. A 

 watery fluid can soon be observed round these fragments in 

 the oil. Finer and coarser droplets appear, and not in- 

 frequently fragments of the enclosed substance are extruded 

 from the surface of the drop and dissolve in the sur- 

 rounding water ; or at times even the watery fluid that has 

 made its appearance in the oil pours out eruptively into the 

 surrounding water. The fact that a lively and fairly 

 regular exchange takes place by diffusion between the oil 

 mixture and the water is shown by .the active streaming 

 movements of the latter, which can be well followed if Indian 

 ink is mixed with it. I have followed these movements to 

 some extent in some drops of oil mixture prepared with 

 common salt, and can report on them as follows. After 

 transferring the drop of mixture to the slide it is soon 

 ^ observed that in the higher regions the water streams from 

 '\ all sides towards the drop, and, on the contrary, in the lower 

 regions, i.e. on the slide itself, the water streams away from 

 the drop radially. These currents gradually become slower, 

 but can be followed for about twenty minutes, when they either 

 die down or continue very feebly. It was often observed 

 that the upper current, at first purely radial in direction, 

 gradually changed in such a way that at one region of the 

 drop an out-streaming current was formed even at the higher 

 level, while at other places the in-streaming currents con- 

 tinued, but had slightly altered their directions in consequence. 



The above-mentioned streaming movements admit of an easy 

 explanation. The water immediately surrounding the drop takes 

 up salt solution, either by diffusion or by the direct emergence of 

 single particles, becomes specifically heavier thereby, sinks down 



