1903.] Solids under Mechanical Disturbance. 221 



The liquid tends to spread over the face in all directions, no doubt 

 owing to the perfect freedom from dust and grease of the surface ; but, 

 as far as possible, it is desirable to confine the etching action to one 

 spot so that a comparison between the etched and unetched portions 

 may be more readily made. Slight etching resulted from the treat- 

 ment, but the etched surface, though slightly undulating and irregular, 

 showed no new structure or markings. When the experiment is 

 repeated a number of times it is generally found that the surface, 

 though slightly lowered where the acid has acted, is otherwise un- 

 altered. The face of the crystal was now firmly stroked a few times 

 in one direction with the point of the fore-finger covered with clean 

 soft wash-leather. Under the microscope the surface was still smooth 

 and unmarked by the stroking. A drop of the dilute acid was placed 

 on a part of the stroked face which had not been touched by the 

 previous application of acid. The drop flattened to a hemisphere but 

 did not spread. After 10 to 15 seconds it was removed as before. 

 The pit produced had well-defined edges and the flat bottom was 

 covered with furrows and ridges running in the direction of the strokes 

 with the finger. 



This experiment was repeated in a variety of ways and on all of the 

 three parallel sets of faces. The direction of the strokes was varied 

 so as to make a number of different angles with the cleavages. In 

 every case the result was the same, even a single stroke showed un- 

 mistakably on the etched surface. 



Plate 9, fig. 1, shows a part of the etched surface. The unetched 

 portion has the smooth glass-like surface of the natural face, only a 

 few very faint scratches being visible under the most searching 

 examination. Faint cleavage lines crossing the. face indicate that the 

 stroking had not been parallel with any of the natural cleavages. 



The number and depth of the flow lines is influenced by the amount 

 of the stroking and by the pressure exerted by the finger. Flow lines 

 were distinctly developed by a single firm stroke in which, it was 

 roughly estimated, the pressure exerted did not exceed 4 Ibs. per 

 square inch. In this case the flow lines were obviously fewer than 

 they were when a number of strokes had been given. 



Some indication of the depth to which the molecular disturbance 

 has penetrated is given by the additional time required for the acid to 

 dissolve and remove all traces of the disturbance. If the acid is left 

 on the surface for a sufficiently long time, for instance, with the acid in 

 question, for 30 to GO seconds, no trace of the flow lines remains, and 



