Spread of Liquids on Solid Bodies. 331 



angle of the water against the glass remains almost unaltered. 

 Should an alteration take place, it is sometimes positive, some- 

 times negative, and seldom amounts to more than 1°. Con- 

 sequently I could not detect any change of the surface of 

 contact of glass and water. 



5. Water behaves towards other solid bodies — quartz, calc- 

 spar, mica, &c. — similarly as towards glass. Here also the 

 edge-angle is smaller as the surface of the solid body is cleaner. 



The surface of glass is never obtained clean by rubbing it 

 with a clean linen cloth and with alcohol, nor by longer im- 

 mersions in alcohol. The best course is to treat the glass 

 with hot concentrated sulphuric acid, wash this off with dis- 

 tilled water, let it lie a considerable time in clean water to 

 remove the last traces of acid, then take the plate with pla- 

 tinum tongs and dry it in the warm current of air above the 

 colourless flame of a Bunsen burner. 



The plates are allowed to cool upon a clean watch-glass in 

 a large clean glass jar covered with a glass plate. 



There certainly remains after this process, at the edge of the 

 last drop that dried up, a little of the glass which had been dis- 

 solved in the water ; and this coating modifies the edge-angle. 



In a similar manner cut plates of quartz may be cleaned. 



In the case of selenite, mica, calc-spar, and topaz, fresh 

 surfaces of cleavage are used. 



The acute edge-angle of water upon the substances men- 

 tioned is generally the greater the longer the time which has 

 elapsed since the cleansing or the formation of the clean sur- 

 face, during which the solid bodies condense upon their surface 

 gases or vapours from the air*. Even a few seconds suffice 

 to allow the influence, which always increases the edge-angle 

 of the water, to become recognizable. Topaz appears to me 

 most sensitive ; less sensitive are calc-spar, glass, selenite, mica, 

 and quartz, which last substance keeps a clean surface the 

 longest. 



Since I found it impossible to split the substance without 

 touching it on the edge with the finger, the freshly cloven 

 surfaces may probably in consequence have been also soiled. 



I have determined the edge-angle with the cleanest possible 

 surfaces specially for water and olive- oil, and for water with a 

 surface rubbed with olive-oil and a clean cloth, or greasy. 



The figures given are the mean of several measurements. 

 Under " min. " are placed the smallest values which I have 

 found in these measurements. 



* Biess has formerly remarked (Reibungselectricitat, vol. ii. p. 220) that 

 a drop of water remains stationary upon an old surface of mica, but flows 

 at once over a fresh one and wets it. 



