206 EXPERIMENTS ON ADHESION AND COHESION. 





manifested between a solid and a liquid. A finger, a 

 glass-tube, a piece of sealing-wax, or a pencil immersed 

 in mercury and withdrawn, will show no trace of the 

 liquid upon their surface ; nor will adhesion be mani- 

 fested if mercury be thrown upon a clean table ; globules 

 will be formed in the same manner as when water is 

 thrown upon a table covered with lycopodium. The 

 metals alone, among common substances, are wetted by 

 mercury, though even among them iron is an exception, 

 and the rest are wetted only when their surfaces 

 are perfectly clean. Nevertheless it may be shown 

 that adhesion exists even if the solid surface is not 

 moistened by a liquid. A small square or round plate 

 of glass is suspended by the hook attached to the short 

 scale-pan of the balance and counterpoised. A vessel 

 containing some mercury is placed underneath the glass 

 plate so that the latter may just touch the surface of 

 the mercury, as shown in fig. 147 ; the plate will adhere 

 to the liquid, and weights amounting to several grammes 

 will have to be placed in the other scale-pan to detach the 

 plate again. If water be used instead of mercury, only 

 about a third of the weights used in the experiment 

 with mercury will be needed to effect the separation oi 

 the plate from the liquid ; but it would be erroneous to 

 conclude from this that the adhesion between glass and 

 mercury is three times as great as that between glass 

 and water, for the weights used in the latter experiments 

 have in fact not overcome the adhesion between glass; 

 and water; the glass plate is covered with drops oi 

 water, and the force represented by the weights has been 

 chiefly used for detaching some of the liquid from the 

 remaining mass. The experiment proves no more than 



