LAWS OF CHEMICAL COMBINATION. 43 



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 *he combination of atoms into molecules ; 

 the properties 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 

 had been mixed together in any other proportion than the one mentioned, and 

 which corresponds to the atomic weights of both elements, the excess of one 

 will be left undisturbed and uncombined. 



Law of multiple proportions. If two elements, A and B, are 

 capable of uniting in several proportions, the quantities of B which 

 combine with a fixed quantity of A bear a simple ratio to each other. 

 Thus A may combine with B, or A with 2 B, or A with 3 B, etc. 



This law was discovered at the beginning of the present century, 

 when it was found that the ratio of carbon to hydrogen in olefiant 

 gas, C 2 H 4 , is as 6 to 1, in marsh gas, CH 4 , as 6 to 2, and that the 

 ratio of carbon to oxygen in carbon monoxide, CO, is as 6 to 8, in 

 carbon dioxide, CO 2 , as 6 to 16. 



These and similar instances led to the discovery of the law of 

 multiple proportions, and it was this law which led Dalton, in 1804, 

 to the adoption of the atomic theory. In thinking and reasoning 

 about this law, he could find no other explanation than that there 

 must be small particles of definite weight which combine with each 

 other, and to these small particles he gave the name atoms. 



As a very good example illustrating the law of multiple proportions may be 

 mentioned the five compounds formed by the elements nitrogen and oxygen^ 

 which compounds have the composition N 2 O, N 2 2 , N 2 3 , N 2 O 4 , and N 2 O 5 , 

 respectively. In these compounds we find 16, 2X16, 3X16, 4X16, and 5X16 

 parts by weight of oxygen in combination with 28 parts by weight of nitrogen. 



