94 PRINCIPLES OF CHEMISTRY. 



starch may be easily distinguished. The mixture thus produced is n 

 mechanical mixture of. molecule clusters. 



It is somewhat different when two substances, for instance two 

 metals, are fused together, or when two gases or two liquids (oxygen 

 and nitrogen, water and alcohol) are mixed together, or when finally 

 a solid is dissolved in a liquid (sugar in water). In these instances 

 no separate particles can be discovered even by the microscope. The 

 mixtures thus produced are mixtures of molecules. Such mixtures 

 always exhibit properties intermediate between those of their constitu- 

 ents and in regular gradation according to the quantity of each one 

 present. The proportions in which substances may be mixed are 

 variable. 



In a true chemical compound the proportions of the constituent 

 elements admit of no variation whatever ; it is not formed by the 

 mixing of molecules, but by the combination of molecules; the prop- 1 

 erties of a compound thus formed usually differ very widely from 

 those of the combining elements. 



Powdered iron and powdered sulphur may be mixed together in many 

 different proportions. If such a mixture be heated until the sulphur becomes 

 liquid, the two elements, iron and sulphur, combine chemically, but they do so 

 in one proportion only, 56 parts by weight of iron combining with 32 parts by 

 weight of sulphur, to form 88 parts of sulphide of iron. If the two substances 

 are mixed together in any other proportion than the one mentioned, the excess 

 of one will be left uncornbined. 



Law of multiple proportions. While two or more elements 

 may unite in certain definite proportions to give one definite com- 

 pound, it does not follow that, under other conditions, they may not 

 unite in other proportions to give another entirely different com- 

 pound, but still perfectly definite in its composition. Many such ex- 

 amples are known in chemistry. Copper unites with oxygen in two 

 proportions, forming two distinct oxides. Tin does the same. There 

 are four different compounds of the elements potassium, chlorine, and 

 oxygen, and nitrogen and oxygen unite in five different ways. In 

 1804 John Dalton, of England, by a study of such multiple com- 

 pounds, proposed the law of multiple proportions. He had studied 

 the composition of two gaseous compounds of carbon and hydrogen, 

 and found one (olefiant gas) to contain 6 parts of carbon to 1 part of 

 hydrogen, while the other (marsh gas) contained 6 parts of carbon 

 to 2 parts of hydrogen. Upon what seems now to be a slight basis, 

 Dalton put forth his law, which, however, has been verified by every 



