80 THE REALITIES OF MODERN SCIENCE 



volume of air or other mixture is the sum of the pressures 

 which each component gas would exert if contained 

 alone in a similar volume. Dalton was, therefore, 

 able to visualize a mixture of gases as a purely mechan- 

 ical mixture of small particles, all those of each con- 

 stituent gas being alike. 



He called all such particles " atoms" and failed to 

 make the modern distinction between the molecular 

 and atomic particles. Such a distinction was first 

 pointed out by Avogadro in 1811. The classification 

 of Avogadro, however, was not generally accepted 

 during the next few years, and considerable confusion 

 resulted. In fact, the confusion as to the concepts of 

 atom, molecule, and atomic weight gave rise to such 

 conflicting theories and methods of expressing chemical 

 composition that the development of the science was 

 impeded. A conference was therefore called at Karls- 

 ruhe in 1860 to discuss the various hypotheses. A 

 paper containing the researches and theories of Canniz- 

 zaro, a professor at Rome, which had been published 

 in 1858, was brought to attention. With the acceptance 

 of his conclusions modern chemistry became established. 



If we accept the idea that the molecules of any kind 

 of matter are similar and are formed by the combina- 

 tion of atoms of different elements, all the atoms of 

 any element being alike, we reach at once certain 

 conclusions. Thus, suppose that we ask what possible 

 compounds may be formed of two elements, say A and 

 B. The simplest molecule would be formed by one 

 atom of A and one of B. Such a compound molecule 

 we might then symbolize as AB. The molecule might 

 be formed of one atom of A and two of B, in which case 



