222 Mr. G. T. Beilby. Surface Flow in Crystalline [May 23, 



the etched surface presents the same appearance as the etched surface 

 of an untouched face, the limits of the disturbance have been reached. 

 The depth to which the disturbance penetrates is not much affected 

 by continued stroking. The influence of the stroking is therefore 

 limited to a very thin surface film, but within this limit it is quite 

 unmistakable. If a stroked and etched surface is further stroked its 

 details become rounded and glazed over. 



The nature of the structural change which has taken place in the 

 surface layer of the Iceland spar as it is disclosed by etching will be 

 referred to in another connection. For the present it is proposed 

 simply to discuss these observations in their immediate bearing on the 

 surface flow of crystalline solids. The most striking facts brought 

 to light by this experiment are ; that a solid structure can be pro- 

 foundly modified below its surface while the surface itself appears 

 intact; and further, that this change can be brought about by so 

 gentle an agent as the softly covered finger. 



There appear to be two directions in which the explanation of the 

 first fact may be sought ; either there has been a transmission of 

 stress from the surface which has produced a state of strain along the 

 lines followed by the moving points of the polishing material ; or there 

 has been a movement of translation of the molecules resulting in flow 

 of the substance similar to that which is seen at the surface of 

 crystalline metals. 



The first of these suggestions finds very little support from the facts 

 of the case. The flow lines are equally deep and distinct whether 

 they run parallel to or at an angle with the cleavages. If the flow 

 lines exist in the stroked but unetched surface as hidden clefts it might 

 reasonably be expected that the chief effect of the stroking would be 

 felt along the natural cleavages ; but this is not the case. Again, the 

 prompt and uniform action of the solvent in first removing the wJwle 

 of the uppermost layer of the surface, suggests that this covering film 

 is fairly homogeneous and that it is not a kind of mosaic with concealed 

 joints. This remark applies also to the kind of under-structure 

 disclosed by the removal of the surface ; the rounded details of the 

 ridges and furrows do not in the least suggest the clean sharp planes 

 and angles of cleavages. 



It seems, on the whole, most probable that the surface molecules are 

 set in gliding motion by the moving leather, so that they form an 

 extremely thin film of fluid among which the larger particles or 

 aggregates are dragged, causing the furrows which are afterwards 

 disclosed by etching. The film of gliding molecules will be subject to 

 surface tension, which would account for the perfectly smooth surface 

 which is left by polishing. The fluid film when left to itself would 

 naturally close over the asperities like an unbroken skin, thus covering 

 up all trace of the disturbance caused by the stroking. 



