

EXPERIMENTS ON ADHESION AND COHESION. 207 



that the adhesion between glass and water is greater 

 than the mutual attraction of the particles of water, and 

 generally : A solid body is moistened 

 by a liquid if the cohesion of the 

 liquid is less than the adhesion 

 tween it and the solid ; but if the. 

 cohesion is greater than the adhe- 

 sion, the solid will not be moistened 

 by the liquid. 



The glass plate should have a size of 2 

 or 3 cm , and you may either cut it circular 

 with pastille, or have it cut square by a 

 glazier. 



The mercury used in this and in many 

 other experiments must be free from 

 impurities, for only pure mercury re- 

 tains its bright surface permanently ; im- FlG - 14 ? (i real size }- 

 pure mercury becomes tarnished on the surface, has a dull appear-! 

 ance, and sticks to glass and other bodies, covering them with a 

 grey layer. Mechanical admixtures, such as dust, are easily got 

 rid of by pouring the mercury through a funnel made of a piece 

 of blotting paper, which is twisted into the shape of a cone, with 

 a fine aperture at the point ; the impurities remain behind, and 

 adhere to the paper. The last portion of the me cury in the funnel 

 will not run easily through the fine aperture ; it should be dropped 

 separately into a small vessel, and kept for some experiments 

 which, as will be seen farther on, do not require mercury in a 

 perfectly pure state: Mercury which has become moist by water 

 may be dried by blotting paper ; the larger drops of water are as 

 far as possible sucked up by scrips of paper, and the remainder is 

 filtered through a cone of blotting paper. From the property which 

 mercury possesses of dissolving most metals, it often contains im- 

 purities which are more difficult to deal with than those previously 

 mentioned. Mercury which is rendered impure by the presence of 

 other metals leaves a considerable residue on the paper filter, and 

 exhibits for a short time after filtration the bright lustre of the pure 

 metal ; but it becomes soon covered with a grey film, and if the quan- 

 tity of foreign substances is considerable, it loses its mobility and 

 the surface becomes covered, even immediately after filtering, with 

 numerous thin folds and ruflfles. The purification of such mercury is] 



