326 Prof. G. Quincke on the Edge-angle and 



Except in the case of bisulphide of carbon, where an impos- 

 sible value of the edge-angle is found in the capillary tubes, 

 the values of the edge-angle as determined by both methods 

 agree with each other. 



I obtained the same result with aqueous solutions of salts 

 and with alcohol. For these I have combined, from innumer- 

 able experiments, the mean values of the edge-angles 6 for 

 air-bubbles and S for capillary tubes in the last two columns 

 of Table XI. of a previous communication*. These exhibit 

 most discrepancy between 20° and 30°. 



Greater discrepancies between 6 and 3 are shown only with 

 solutions of KC1, MgCl 2 , CuS0 4 , NaN0 3 , KN0 3 , and especi- 

 ally of carbonate of potash. 



If these discrepancies might also have their origin in acci- 

 dental impurities, it appeared to me nevertheless to be desirable, 

 instead of these convenient estimations, to bring about direct 

 measurements of the edge-angle, the more so as a much greater 

 accuracy may be attained by the latter f. 



3. In order to measure directly the edge-angle 0, which the 

 extreme portion of a free fluid surface makes with the level 

 surface of a solid body, I employ the following method of 

 reflexion: — A clean thread of glass (tubing), newly drawn out 

 in a flame, is bent into a little siphon ; the ends are cut off with 

 a clean file, and the siphon is placed in a glass which stands 

 upon a clean horizontal plate of plate-glass, G x . Pass a small 

 spirit-flame below the bend of the siphon, and the limb of the 

 siphon sets itself exactly vertical. 



If the glass be filled with a liquid, it climbs by capillary 

 attraction up the glass thread as high as the bend ; and at the 

 opening of the vertical tube of the siphon bounded by sharp 

 edges (see fig. 1) drops are formed with clean surfaces. The 

 volume of these drops is almost independent of the velocity 

 with which the drops are formed, and is equal to half the 

 specific cohesion a 2 of the liquid in question multiplied by the 

 periphery of the tube-wall on which the drop forms. 



In most cases the drops are formed on the outer wall of the 

 siphon-tube, so that it is possible, by the selection of glass 

 threads of suitable internal and external diameters (usually 0*5 

 to 1 millim.), to get drops of suitable size to follow one another 

 at intervals of from about 1 to 30 seconds. By shifting the 



* Pogg. Ann. clx. pp. 371-374 (1877). 



t For mercury and transparent solid "bodies the method of two reflexions 

 formerly contrived by me is to he preferred (Pogg. Ann. ev. p. 40. 1858). 

 The changes of the edge-angle observed at that time I think must chiefly 

 be ascribed to the oil-vapours which expanded in the apparatus exhausted 

 of air, as then used. 



