104 PRACTICAL LESSONS IN SCIENCE. 



Another difference between a physical mixture and a chemical 

 combination is that the former may consist of any possible 

 proportion of the ingredients, while in the case of chemical com- 

 binations the proportions are definite. Take the case of sulphur 

 and iron; they may be mixed in any proportion, but they com- 

 bine in the proportion of 56 parts of iron to 32 parts of sulphur. 

 If there is either too much sulphur or too much iron, the surplus 

 does not enter into the combination. In the case of oxygen and 

 sulphur the proportion of the chemical combination is 32 of oxy- 

 gen and 32 of sulphur. Experiments with many substances have 

 led chemists to the conclusion that chemical combinations always 

 occur between definite weights of substances. In some cases the 

 weight of a substance may vary in different combinations, but in 

 every such case some number can be found which shall express 

 the combining weight, or the weight will be some simple multiple 

 of that number. 



Some of the ancients considered matter as made up of atoms, 

 but the idea was never more than a speculation. And when it 

 had been proved that chemical combinations were in definite 

 proportions, the old idea of atoms assumed a different position. 

 Dr. Dalton, who, early in this century, discovered and announced 

 the law of definite and multiple proportions, saw that the con- 

 ception that matter is made up of indivisible particles or atoms 

 might have some connection with this law. As a result of his 

 studies he conceived that the law might be explained by assum- 

 ing that all matter consists of indivisible, unchangeable par- 

 ticles, or atoms; that atoms of the same kind have the same 

 weight, while those of different kinds have different weights, and 

 that the combining weights of chemistry represent these atomic 

 weights. The supposition is that atoms of iron and atoms of 

 sulphur unite to form molecules of the compound substance, 

 sulphide of iron, so of the oxygen and sulphur, and other chem- 

 ical combinations. Atoms are considered the units of simple 

 substances or elements, and molecules, made up of atoms, are 

 the units of compound substances. The artomic theory, and the 

 law of definite and multiple proportions in chemical combinations 



