52 THE ROYAL SOCIETY OF CANADA ' 



angle through which the mirror is turned is the supplement of the angle 

 of contact of the mercury and glass or the value of the angle measured 

 in the air or the other liquid. 



Measurements on mercury and glass with air as the third sub- 

 stance yielded somewhat discordant results, as such results usually 

 are. The following taken at different times gives the results for freshly 

 prepared surfaces: 



147° 36' 144° 5' 146° 19' 143° 29' 144° 10' 142° 59' 

 These readings represent angles in the mercury. 



Next readings with pure distilled water on the mercury were taken. 

 Here a small weight was placed on the cover glass to prevent its 

 floating off. With pure distilled water and clean glass, there is an angle 

 of contact. The following are results for glass, mercury and water : 



4° 7' 5° 45' 5° 13' 5° 11' 4° 46' 

 with occasionally other results rising as high as 11° which are evidently 

 due to traces of dirt. These angles are measured in the water. 



With a strong sulphuric acid solution, the observations are easy, 

 and always yield an angle O. Starting with a 25 per cent solution, 

 observations were taken with constantly diminishing concentration 

 with the following results : 



Above 2 per cent 



2 per cent 5° 59' 6° very small 



With the 2 per cent solution the observations are very difficult. There 

 is not the easy decision as in the higher concentration. Somewhere 

 between a 2 per cent concentration and pure water, the angle passes 

 from 0° to about 5°. The exact point of change from zero angle has 

 not been determined but is probably for a concentration of less than 

 ^ per cent. 



In the case of three pure liquids in contact along a line, it has been 

 shown that the Neumann triangle is impossible, but, with a solid and 

 two liquids, equilibrium is possible and there may be an angle of 

 contact. In the case of a mercury-water surface in contact with glass, 

 for example, there is an angle as shown above. If Ti is the surface 

 tension of the water-mercury surface,- T2 of the glass-mercury surface 

 and T3 of the glass-water surface, the equation of equilibrium is 



To — T3. 



T3 -f Ti cos 6 = T2, so cos 6 = — "—= 



1 1 



If T3 — T2>Ti, equilibrium is impossible and the angh of 

 contact has become zero. This, in general, represents a condition of 

 instability. For the water and mercury on glass we have stability 

 With the acid-mercury surface, at least for all but the smallest con- 

 centrations, the condition is one of instability. In this case, the acid 



