SECT. ii. COHESION AND CAPILLARY ATTRACTION. 25 



this store, the store itself being eternal and unchange-* 

 able.' 2 



Cohesion is a force which acting at inappreciably 

 small distances unites atoms and molecules of the same 

 kind into solids, liquids, and aeriform fluids, exactly ac^ 

 cording to the law of the conservation of energy ; for it 

 requires the very same amount of force to dissolve their 

 union as to form it. Cohesion varies with temperature 

 both in simple and compound bodies, for metals can be 

 fused and vaporized by artificial heat, and ice becomes 

 water and aqueous vapour as the seasons change from, 

 winter to summer. 



In solids the force of cohesion is so strong, that their 

 atoms and molecules always retain their respective places ; 

 that power is so weak in liquids, that their atoms and 

 molecules are capable of motion among themselves, and 

 in gases and the ethereal medium the atoms are free and 

 have no cohesion whatever. The resistance offered by 

 substances to compression is an equal and contrary force* 

 The reciprocal attraction between solids and liquids 

 in capillary tubes is a case of cohesion. If a glass tube 

 of extremely fine bore be plunged into a glass of water 

 or alcohol, the liquid will immediately rise in the tube 

 above the level of that in the cup, and the surface of the 

 little suspended column will be a hollow hemisphere. If 

 on the contrary mercury be the liquid, it will not rise so 

 high in the glass tube, and the surface of the little 

 column will be a convex hemisphere. There is a reci- 

 procal attraction between the glass tube and the liquid, 

 and another between the particles of the liquid itself; 

 and the effect is produced by the difference between the 

 two. In the first case the attraction of the glass isj 

 greater than that of the liquid, and in the second it\ 

 is less ; hence the water rises higher in the tube 

 % - 



7 Professor Helmholtz. 



