RADIAL STRIATION 43 



plainly seen under the microscope that the particles of 

 lamp-black at the superficial region of the oil-drop, down to 

 a greater or less depth, arranged themselves after a short 

 time in rows, all directed radially to the free surface of the 

 drop. As a result the marginal zone presented a beautiful 

 radiate appearance to a greater or less extent. As has 

 been mentioned, this phenomenon appears in ordinary drops 

 of olive oil, usually with great distinctness. I obtained it, 

 however, still better and more distinctly when some particles 

 of anhydrous calcium chloride were enclosed in the oil-drops. 

 Crystals of nitre served in the same manner ; on the other 

 hand, the inclusion of drops of glycerine in the oil was not so 

 good. With enclosures of the kind named a radiate striation 

 could often be observed round the drops of solution of nitre 

 or calcium chloride, which had become formed round the 

 particles enclosed in the oil. The fact, therefore, that these 

 radiate appearances are also strengthened by the diffusion 

 processes, which are doubtless set up by the particles enclosed 

 in the oil-drops, appears to support the view expressed above 

 as to the cause of the striation in the drops of oil-lather. 

 If it were possible to introduce into the foam-drops particles 

 of some substance that attract water strongly, I think that 

 radiate appearances would appear much more marked. Un- 

 fortunately no means for effecting this have as yet been in 

 my power, since the drops break up on raising the cover 

 glass. 



The very same radial striation can, moreover, be easily 

 observed in drops of paraffin oil mixed with fine lamp- 

 black, and it therefore does not depend directly on the 

 chemical quality of the oil. I have frequently repeated 

 of late the experiments with paraffin oil which were per- 

 formed at an earlier date, and have again observed radiate 

 appearances very well and very distinctly. Indeed I have 

 obtained the impression that they develop quicker and 

 better in paraffin oil than in olive oil. Since radiate 

 appearances can also appear under some circumstances as 

 optical effects round air-bubbles, I may lay special emphasis 

 on the fact that the radial striation in the oil or froth- 

 drops makes its first appearance gradually, and frequently 



