ANALYSIS OF COMPLEX MATTER 145 



95. Dalton's Atomic Hypothesis and its later Develop- 

 ment. The conception of all matter as being constructed of 

 very minute separate particles, atoms, prevailed among Greek 

 philosophers at a very early date. Their opinions, however, 

 have little in common with modern views. In the year 1808 

 John Dalton published his ' New Principles of Chemistry.' He 

 embodied the older guesses in a systematic hypothesis, which 

 aimed at a complete explanation of the fact then known, that 

 compounds have an invariable composition. According to his 

 views, the formation of a chemical compound is an action 

 taking place between indivisible atoms. Since all the atoms 

 of the same kind of matter are absolutely alike, the relative 

 masses of two substances combining will be the same as the 

 relative masses of the atoms. These relative masses were all 

 expressed in terms of the smallest as unity, which was found to 

 be that of hydrogen. The numerical values thus obtained 

 were called atomic weights. 



An example will make Dalton's hypothesis clearer. Water 

 is proved to contain always the same relative quantities of 

 oxygen and hydrogen, viz. 8 to 1. It has an invariable com- 

 position. Each particle or atom of water will therefore 

 contain 1 atom of oxygen and 1 atom of hydrogen ; and an 

 atom of oxygen is eight times heavier than an atom of hydrogen. 



We are now aware that, if the volumes of oxygen and 

 hydrogen combining to form water are observed, the volume 

 of the hydrogen will be found double that of the oxygen. If 

 Avogadro's hypothesis be true, there will be twice as many 

 distinct particles of hydrogen as of oxygen in water. Dalton's 

 hypothesis, therefore, does not agree wholly with all observa- 

 tions. 



Dalton had observed that, when two substances form 

 more than one compound, the relative quantities in one com- 

 pound bear a simple ratio to the relative quantities in another. 

 For example, he found that a certain quantity of carbon 

 united with a certain quantity of oxygen to form one oxide of 

 carbon, while it formed another oxide by uniting with just 

 double that quantity of oxygen. Likewise, a certain quantity 

 of carbon unites with a certain quantity of hydrogen to form 



