LAWS AND THEORIF:S OF CHEMISTRY. 93 



application of heat we can further subdivide the sulphur, melt, and 

 finally volatilize it ; or we can throw a piece of sulphur into the air, 

 when it will fall back upon the earth in consequence of gravitation ; 

 or we can dip it into water, when it becomes moist in consequence of 

 surface-action. Yet in all these cases sulphur remains sulphur. 



It is entirely different when sulphur enters into chemical combina- 

 tion exerting chemical attraction, for instance, when it burns; this 

 means when it combines with the oxygen of the atmospheric air. In 

 this case a new substance, a disagreeably smelling gas, a compound of 

 oxygen and sulphur, is formed. 



It is^ consequently a complete change in the properties of matter 

 which follows the action of true chemical attraction ; we might define 

 affinity to be a force by which elements unite and new substances are 

 generated. 



It should be noted that affinity does not really explain why chem- 

 ical union takes place, why an attraction between elements exists. 

 It is merely a term that has come into use to express a fact that can 

 be observed ; namely, that elements do unite, but why they do so no 

 one knows. Likewise no one knows why fhe earth attracts bodies, 

 although we say it is due to gravitation. This is simply equivalent 

 to saving that an attractive force exists between the earth and bodies 

 upon it, a fact which anyone can observe. But no one knows why 

 this force exists or its nature. 



6. LAWS AND THEORIES OF CHEMISTRY. 1 



Law of the constancy of composition. This law, also known 

 as the law of definite proportions, was the first ever recognized in 

 chemical science ; it was discovered toward the close of the 18th 

 century, and may be stated thus : A definite compound always contains 

 the same elements in the same proportion; or, in other words, All chemi- 

 cal compounds are definite in their nature and in their composition. 



To make this law perfectly understood, the difference between a 

 mechanical mixture and a chemical compound must be pointed out. 

 Two powders, for instance sugar and starch, may be mixed together 

 very intimately in a mortar, so that it seems impossible for the eye to 

 discover more than one body. But in looking at this powder with 

 the aid of a microscope, the particles of sugar as well as those of 



1 The subject matters of chapters 6, 7, 8, and 9 are grouped together for convenience. It is 

 not intended that they should be taken up in lectures or class work at once after chapter 5, 

 nor in the exact order given here, but each instructor will introduce them in what, to him, 

 seems the most logical sequence. 



