42 CHEMISTRY. 



CHAPTER IV. 



CHEMICAL PHILOSOPHY (CONTINUED). 



37. Forces. The force which binds unlike molecules to- 

 gether is called cohesion; it is this power which gives rigid- 

 ity to solid bodies, and which, though weaker in fluids, pre- 

 serves their particles in contact. The attraction exerted be- 

 tween unlike molecules is called adhesion; this is the force 

 which makes water adhere to solid bodies, and which you 

 Lave already studied in Part I. The power which unites 

 atoms within the molecule is called chemical attraction^ 

 though sometimes spoken of as affinity. To illustrate these 

 different forces, we may say that the molecules of a pane of 

 glass are held together by cohesion ; dip the pane in water, 

 and the water sticks to it by adhesion ; while the atoms of 

 silicon, calcium, potassium, and oxygen (of which the mole- 

 cules of glass are composed) are held together by chemical 

 attraction. Observe that in the attraction of adhesion and 

 of cohesion the particles are merely held together, without 

 producing any change in the nature of the substances which 

 attract each other, however different they may be. But 

 when two substances attract each other chemically, in the 

 union that occurs a change is produced in both. Moreover 

 ordinary attraction operates at all distances, in masses as 

 well as in particles, while chemical attraction operates only 

 when the particles of substances are intimately mingled 

 as we usually say, in actual contact. Note, also, that chem- 

 ical attraction has nothing in common with electrical or 

 magnetic force about which you studied in Part I, for the 



