io PROTOPLASM 



on to the slide, and spreading out upon it, streams in all directions 

 away from the drop, while in the higher regions it is replaced by 

 pure, specifically lighter water, which therefore streams from all 

 sides towards the drop. The correctness of this explanation is 

 confirmed by the fact that the same streamings are also obtained 

 by allowing concentrated salt solution and pure water to approach 

 one another under a cover-slip a process which can be carried out 

 with the retention of a fairly sharp boundary between them. 

 The current in the water, which is directed towards the boundary, 

 goes on in the upper region, while in the lower region the current 

 flows away from it ; on the other hand, in the salt solution the 

 current towards the boundary is below, the current away from it 

 uppermost, since the salt solution of the intermediate region, having 

 become specifically lighter, continually ascends. The streaming 

 movements are distinct but fairly slow. Corresponding stream- 

 ings appear if glycerine is brought in contact with water. 



After the drops of oil mixture, prepared in the manner 

 stated, have stood about twenty-four hours in a damp cham- 

 ber, they become completely opaque and milk-white. The 

 particles of soluble substance have vanished, but here and 

 there larger drops of fluid (vacuoles) are visible in the oil. 

 The closer investigation of drops that have thus become 

 opaque shows that they have become converted into a more 

 or less finely-structured foam throughout their entire mass. 

 On account of their opacity the drops must naturally be 

 pressed out into a thin layer, if it is desired to determine 

 their finer composition. Hence a more suitable method is 

 to clear them up gradually by addition of glycerine to the 

 water under the cover-slip, or rather by replacement of the 

 water by glycerine. One can then follow plainly the way in 

 which the clearing up gradually penetrates from the surface 

 of the drop into the interior, and finally, after a short time, 

 pervades the whole drop equally. The gradual clearance of 

 such a drop of oil-foam by glycerine is a sure proof that the 

 glycerine diffuses through the oil, so that its alveoli become 

 filled after a time with watery glycerine. In consequence 

 of this the froth-drop naturally becomes much more trans- 

 parent, on account of the diminution in the difference of the 

 refractive indices. I defer for the present the more detailed 

 description of the structural relations of foam-drops produced 



