42 PRINCIPLES OF CHEMISTRY. 



of sugar as well as those of starch may be easily distinguished. 

 The mixture thus produced is a 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 inter- 

 mediate between those of their constituents and in regular 

 gradation according to the quantity of each one present. The 

 proportions in which substances may thus be mixed are variable. 



In a true chemical compound the proportions of the con- 

 stituent elements admit of no variation whatever; it is not 

 formed by the mixing of molecules, but by the combination of 

 atoms into molecules; the properties of a compound thus formed 

 usually differ very widely from those of the combining elements. 



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 cen- 

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

 olefiant gas is as 6 to 1, in marsh gas as 6 to 2, and that the 

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

 carbon dioxide 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. 



The law of chemical combination by volume may be stated as 

 follows : " When two or more gaseous constituents combine chemically 

 to form a gaseous compound, the volumes of the individual constituents 

 bear a simple relation to the volume of the product." For instance : 



