18 CHEMICAL PHYSICS. 



one in which there is no current, although the substance of both wires is the 

 same. Many other examples of physical change might be cited. 



Chemistry is the study of those changes in bodies which affect their compo- 

 sition, and in this respect chemical changes differ from all other kinds of 

 changes. Another good and broad definition given by the great Russian chem- 

 ist, Mendelejeff, is the following: Chemistry is concerned with the study of the 

 homogeneous substances or materials of which all objects of the universe are 

 made up, with the transformations of these substances into one another, and 

 with the phenomena which accompany such transformations. When a piece 

 of paper burns, an ash is left, which is altogether different from the original 

 paper. Moreover, if proper care be taken to catch the products escaping dur- 

 ing the burning, water vapor and gases will be found, which are also unlike 

 the paper. These are new substances and the change is, therefore, a chemical 

 one. But at the same time several physical changes will be observed, namely, 

 heat and light. 



When a piece of the metal magnesium is ignited, it burns and leaves an ash 

 entirely different from the metal, being white and brittle. This is a chemical 

 change, but heat and intense light are observed at the same time, which are 

 physical changes. 



When a piece of marble is heated to redness for some time, a substance 

 remains on cooling which, although having the same form as the piece o> 

 marble, is nevertheless entirely different in its composition and properties, anr* 

 is known as quick-lime. When water is poured upon the latter, great heat is 

 produced .and the solid lump falls down to a white powder, known as slaked- 

 lime, whereas marble is not affected by water. By a suitably arranged appa- 

 ratus it could also be shown tfiat an invisible gas is given off when the marble 

 is heated. 



These illustrations will be sufficient to point out the nature of physical and 

 chemical changes, and we may proceed now to the discussion of some element- 

 ary subjects of physics. 



1. FUNDAMENTAL PROPERTIES OF MATTER. 



Matter is anything that occupies space and may be apprehended 

 by the aid of our senses. While there are many thousands of various 

 kinds of matter, possessing widely different properties, yet there are 

 properties in common which belong to 'every kind of matter, and 

 these are known as essential or fundamental properties. The funda- 

 mental properties of matter having a special interest for those study- 

 ing chemistry are : Extension, Divisibility, Gravitation, Porosity, and 

 Indestructibility. 



Extension. The common property of matter to occupy space is 

 known as extension. All bodies, without exception, fill a certain 

 quantity of space ; they all have length, breadth, and thickness. 

 That portion of matter lying within the surrounding surface of a 

 body is called its mass; or we may define mass as the quantity of 



