34 Mr. W. B. Hardy and Lieut. J. K. Hardy 



of condensed gas. There does not, however, appear to be 

 conclusive proof of the existence on all solid faces of such a 

 layer. It would seem therefore that w 7 e can assert nothing" 

 .as to the presence of a layer of foreign matter on a " clean ,y 

 surface of glass in contact with dry air. 



The tangential force was applied gradually about 1 cm. 

 above the centre of gravity of the watch-glass by slowly 

 withdrawing support from the pan. The effect was to 

 cause the watch-glass to rock forward until equilibrium was 

 reached, when the static friction balanced the tractive force. 

 If the latter were increased beyond a certain quantity 

 determined by the nature and state of the surfaces stalic 

 equilibrium was not reached, but the cohesion gave and the 

 w^atch-glass moved bodily forward. The limit of static 

 friction is the force which just fails to bring about this 

 movement forwards. It is called the " threshold value." 



The watch-glasses were of the same pattern and make, 

 but differently weighted. The threshold values in grammes 

 were : — 



Watch-glass. Weight. Pull. 



Pull 

 per gr. wt. 



] 54*25 grms. 46 grms. 0'85 



2 58-5 „ 50 ., 0-85 



3 57-79 „ 51 „ 0-85 



4 ...... 170-2 „ 143 „ 0*84 



For plates of a rather greenish glass — 



5 14*58 grms. 13*5 grms. 0*93 



16-2 „ 11 



6 58-25 „ 54-2 „ 0-93 



7 46-95 „ 45-0 „ 0-96 



Over a considerable range therefore and for clean surfaces 

 the threshold value is, as a first approximation, equal to the 

 total weight multiplied by a constant which is different for 

 different kinds of glass. 



The effect of temperature was tested by altering the tem- 

 perature of the stream of dry dust-free air passing through 

 the chamber, the actual temperature of the surfaces being 

 taken to be that of a thermometer whose bulb was inside 

 the chamber. Over the narrow range available no effect of 

 importance was detected. 



