'Glass 2.. 



on Statu 



' F 



ricti 



on. 



15-5° 



C. 





Pull 49-5 grras 



19* 







„ 49-5 „ 



23* 







„ 50- 



23-7 







» 51- 



35 



In order to be certain that the watch-glass and plate were 

 actually at the temperature of the air in the chamber, each 

 -stage occupied about an hour. The experiment therefore 

 affords striking testimony to the steadiness of the state of 

 clean surfaces for over four hours in clean dry air. 



These observations were undertaken merely to satisfy 

 ourselves that small fluctuations of temperature did not 

 introduce sensible error. 



The forward movement is always accompanied by an 

 :actual tearing of both glass faces. The length of the tear on 

 the plate is of course determined by the distance forward the 

 watch-glass travels. The length of the tear on the watch- 

 glass is much the same, and is caused by the glass rocking 

 backwards when the seized faces give. 



The appearance to the naked eye or hand-lens is that 

 of a fine scratch pointed at one end. A high magnifi- 

 cation (1000 to 2000 diameters), however, shows that there 

 is no regular continuous cutting of the faces. On both 

 watch-glass and plate the scratched line is composed of an 

 irregular collection of very shallow pits and very thin plates 

 which have been torn from the opposite face. The track 

 begins at the pointed end, it rapidly widens, and the sides, 

 which are more or less clearly defined, become parallel. It 

 ends abruptly with a square or bluntly rounded end. At the 

 pointed end where the movement starts, it consists of delicate 

 flakes or pits of the order 1 /n across. 



These features prove that cleaned faces cannot slide over 

 one another. The forces of cohesion come into play and 

 they seize. Once seized, the substance of the glass fractures 

 before the seized faces will give. Very slight normal pres- 

 sure is needed to cause seizing". An unweighted watch-glass 

 cannot be moved on a plate even quite slowly without pro- 

 ducing these characteristic torn tracks, if both faces are 

 really clean. 



The breadth of the track may be as much as 50 fi. Why 

 does it broaden ? Why does it not continue as it begins at a 

 breadth of about 1 yu ? The widening cannot be due to the 

 abrasion of the curved surface since the contact is continually 

 •changing owing to the rocking backwards of the watch-glass. 



D2 



