LESSONS IN PHYSICS. 21 



in a body is its mass, and is expressed in ounces, pounds, etc. If 

 you cut blocks of wood, cheese and lead of the same size you 

 will find their weights to be different; that is, bodies of the same 

 volume may contain unequal quantities of matter. Those bodies 

 which contain more matter in the same volume than others 

 are said to be more dense than the others. Density refers 

 to the quantity of matter in a given volume as compared 

 with some standard. Lead is more dense than iron, because 

 a cubic inch of lead contains more matter than a cubic inch 

 of iron. 



Cohesion is that form of attraction which acts between the 

 molecules of a body, and which resists dividing, breaking, or 

 tearing forces, differing from gravity in that it only acts 

 through small distances and between molecules of the same 

 kind, making the diamond hard, copper tenacious, and iron 

 strong. 



Cohesion varies with the kind of matter, being stronger in some 

 kinds than in others, and it varies on account of what seems to 

 be a different arrangement of molecules in the same kind of 

 matter, as in the case of tempered and untempered steel. In 

 welding iron the pieces must be heated, at least to the plastic 

 state, before the particles can be hammered together so that 

 cohesion will act. 



Adhesion is that form of attraction which holds together 

 molecules of different kinds of matter. It is this form of attrac- 

 tion that makes glue, paint, mucilage and varnish valuable, and 

 is more m.arked between solids and liquids. 



While adhesion is more marked between solids and liquids it 

 does exist between solids and solids, as in the case of dust adher- 

 ing to a wall, and between liquids and gases as well. This may 

 be shown by pouring water from one vessel into another; air 

 adhering to the descending stream is carried with it, as shown 

 by the bubbles of air formed in the water of the receiving vessel. 

 Gases sometimes adhere to solids. If a piece of glass be immersed 

 in water, bubbles will often appear on its surface. Dust, even 

 iron filings, gently spread over the surface of water, will float, 



