22 PRACTICAL LESSONS IN SCIENCE. 



though much heavier than water. A layer of air adhering to the 

 particles of iron, serves as a boat in which the iron floats. 



If a tumbler be placed on the surface of water and then lifted 

 slowly upward, ifc will draw up the water, forming a little hill. 

 After it has been raised from the water, the film of water on the 

 bottom of the tumbler shows that the force of adhesion between 

 the glass and the water was stronger than the force of cohesion 

 between the molecules of water. This can be measured by letting 

 one pan of a balance rest on the surface of water and then add- 

 ing weights to the other pan till the force of cohesion is overcome. 



Capillary attraction is a kind of adhesion which causes some 

 liquids to rise along the sides of solids partly immersed in them 

 and to rise in narrow tubes, or between plates that are brought 

 near each other. By capillary attraction the blotter absorbs 

 ink, the oil rises in the lamp-wick, and doubtless, in the case of 

 paint and glue, these substances actually penetrate the solid by 

 capillary attraction, and when hardened, mechanical interlocking 

 of the particles aids the force of adhesion in making the union 

 firm. This form of attraction does not act the same in all cases. 

 If a glass tube be partly immersed in mercury, the mercury sinks 

 instead of rising in the tube; in general, those liquids which wet 

 the solid will rise, those that do not will be pushed down, and 

 the smaller the tube the higher the liquid will be raised or the 

 farther it will be pushed down. Experiments with two pieces of 

 glass three or four inches square, separated by different distances, 

 shows that capillary force is manifested between plates in the 

 same manner as within tubes. 



The surface of a solid or liquid, is in a state of tension. It is 

 explained as follows : In the interior of bodies, each molecule is 

 attracted equally in all direction, and no tension exists, while 

 at the surface the molecules a re attracted laterally and inward , 

 but not outward, so that there is strain on the surface molecules. 

 It is through this surface strain that cohesion causes small 

 bodies of liquids to assume a spherical form, as drops of water or 

 mercury. When a soap bubble is formed at the mouth of a pipe, 

 and the stem left open, the bubble soon shrinks and expels the 



