10 INTEODUCTION. 



• 



contact, how they press upon one another, and how the power is transmitted 

 from one point to another : these are experiments in Natm-al Pliilosophy, in 

 which every part of the operation is clear to our senses. But when we mix 

 alcohol and water together, or burn a piece of coal in a fire, we see merely 

 the result of these processes, and our senses give us no dkect mformation of 

 the manner in which one particle of alcohol acts upon another particle of 

 water, or how the oxygen of the air acts upon the coal. These are experi- 

 ments in Chemistry, in which we can not perceive every part of the operation 

 by means of our senses, but only the results. Had there been but one kind 

 of substance or matter in the universe, the laws of Natural Philosophy would 

 have explained all the phenomena or changes which could possibly take 

 place ; and as the character, or composition of this one substance, could not bo 

 changed by the action of any difterent substance upon it, there could be no 

 such department of knowledge as Chemistry. 



4. The term Physics is often used instead 

 b^'lheTerm ^f tlic teiiB Ncitural Philosophy, both having 

 Physics? ii^Q gjinie general meaning and signification. 



It is also customary to speak of " Physical 

 Laws/' " Physical Phenomena," and " Physical Theories," 

 instead of saying the laws, phenomena, and theories of 

 Natural Philosophy. 



5. A Physical Law is the constant relation 

 Physical Laws wliich cxlsts bctwecn any phenomenon and its 



and Theories ? j -r-> m • • i • 



cause. A Physical Theory is an exposition 

 of all the laws which relate to a particular class of 

 phenomena. 



Thus, when we speak of the "theory" of heat, ov of electricity, we have 

 reference to a general consideration of the whole subject of heat, or light, or 

 electricity; but when we use the expression a "law"' of heat, of light, or of 

 electricity, we have reference to a particular department of the whole subject. 



