12 PROTOPLASM 



black and opaque (in transmitted light), so that a dark 

 somewhat irregular margin completely surrounds the drop 

 like a girdle (see Fig. 1). The phenomenon does not per- 

 ceptibly proceed with greater 

 intensity or speed, if the olive 

 oil is heated beforehand for 

 some time on the water-bath 

 over sugar or common salt ; 

 hence, any solution of these 

 substances in the oil is with- 

 out direct influence. On the 

 other hand, the production of 

 froth takes place quite strongly, 

 rig. i. if some few crystalline fragments 



of salt or sugar are enclosed 



in the oil-drops. Drops of fluid then form as a rule round 

 the fragments of salt ; but curiously enough, this does not 

 occur at times round certain of the fragments, and moreover, 

 not round those of sugar. The oil-drop becomes frothy in 

 exactly the way described above, but this process appeared, 

 however, as has been said, to go on more intensely than in 

 the pure oil-drops (see Fig. 1). In the vicinity of the drops 

 of fluid, which had formed round the salt crystals, there 

 appeared to be in no way any special crowding of the froth 

 droplets, which were concentrated, as depicted, more es- 

 pecially in the marginal zone. The explanation of the 

 intense crowding of the droplets of froth in the lower 

 marginal region of the oil-drop may be somewhat as follows. 

 The formation of the droplets depends, without doubt, on 

 water being taken up into the oil, which takes place in a 

 way that will be treated more in detail later. As a 

 consequence, the formation of droplets will occur principally 

 in the marginal region of the oil, bordering on the water. 

 But since the droplets consist of a watery solution, and 

 hence are specifically heavier than the oil, they will sink 

 gradually in the oil-drop, and, as a result, collect principally 

 in the region of its lower margin. Since the droplets when 

 sinking press against one another, they cause, if the conditions 

 are suitable, the formation of a froth, in a similar manner 



