210 LECTURES. 



film of soap-water shown by the load of cotton which it will support. 



The molecular attraction of soap- water shown to be less than that 

 of pure water. Effect of salt in the water. 



Explanation of the different consistencies of bodies, from perfect 

 rigidity to perfect liquidity. Steel at one extremity of the scale and 

 alcohol or ether at the other. 



Difference of the tenacity of sealing wax in the cold and heated state. 



Phenomena exhibited in pulling apart rods of metal of different de- 

 grees of rigidity. 



(90.) Explanation of the development of the increase of the contrac- 

 tile force by curving the suri'ace. 



The molecules, on account of the curvature, are placed in a position 

 more favorable to the action of the attracting force. 



The contractile force increases directly as the curvature, and the 

 resultant is in the direction of the radius of the circle of curvature. 

 Explanation of this, by means of a diagram. 



(91.) Illustrations oj the effect of the contractile force. 



Small bubble made to expand a large one. 



Apparent elasticity of a bubble. 



Phenomenon of the^breaking of a cylindrical bubble. 



Water poured through a bubble. 



Method of forming concentric bubbles. 



Form of a drop of water. Without weight it would be perfectly 

 spherical. Cause of the incurvature of the neck of a pendent drop. 



Explanation of the weight required to flatten a globule of mercury 

 — why two drops of mercury rush together. 



Explanation of the apparent elasticity of a drop of v/ater rebound- 

 ing from the surface of a solid. 



Mercury sustained in a cup of wire gauze. Also water supported 

 in the same manner. 



Effect of wetting the under surface. 



Fine needles made to float on the surface of water. 



Feet of insects which walk on the water ; sink when the feet are wet 

 with alcohol. 



In all these cases a curvature of the surfece is produced which de- 

 velops the contractile force. 



Capillary Attraction, 



(92.) Under this head is classed a set of phenomena belonging to 

 molecular action, among which the ascent of liquids in capillary tubes 

 is the most conspicuous, and hence the name. 



When a plate of glass is plunged vertically into a vessel of water 

 the liquid rises along the surface and covers it to an indefinite height 

 with an exceedingly thin film. On the surface of this film another 

 film rises, and so on until the weight of the accumulated water becomes 

 equal to the elevating force. 



(93.) The thickness of the glass does not affect the result, hence the 

 force is limited in its action to insensible distances. 



