146 ELEMENTS OF LABORATORY WORK 



one compound, while it forms another compound by uniting 

 with double the quantity of hydrogen. An atom of one oxide 

 of carbon, he said, contained one atom of carbon united with 

 one atom of oxygen, while an atom of the other oxide con- 

 tained an atom of carbon united with two atoms of oxygen. 

 He similarly explained the structure of the two compounds of 

 carbon and hydrogen. 



The smallest particles of a compound are now called mole- 

 cules, the word atom being used to denote the ultimate par- 

 ticles of elementary matter. The structure of a molecule of a 

 compound cannot be decided until many more investigations 

 have been made. We may say that it is separable into unlike 

 parts, if the atomic hypothesis have any truth in it ; but we 

 cannot yet say how many of these parts there are, although 

 we may assert that there must be at least as many as there 

 are individual constituents. 



By observing the relation that exists between the volumes 

 of combining gases and the volumes of the gases resulting 

 from their union, we gather that the molecules of elementary 

 gases must be divisible into at least two parts. It is to these 

 parts that the term atom is now given. For example, equal 

 volumes of the gases, hydrogen and chlorine, unite to form a 

 compound, hydrogen chloride, which is a gas occupying the 

 same volume as its components together occupied before com- 

 bination. That is, if we accept Avogadro's hypothesis, one 

 molecule of hydrogen and one molecule of chloride form two 

 molecules of hydrogen chloride. But each of these molecules 

 of hydrogen chloride contains some of the substance hydrogen, 

 and some of the substance chlorine. In other words, the 

 molecule of hydrogen is now divided into two portions, and so 

 is the molecule of chlorine. In accordance with the system of 

 symbolic notation used in Chemistry, H 2 is written for the 

 molecule of hydrogen, while H represents an atom of hydrogen. 

 The reaction taking place is represented by the following 

 chemical equation : 



H 2 + C1 2 =2 HC1. 



Or, one molecule of hydrogen unites with one molecule of 

 chlorine, forming two molecules' of hydrogen chloride. 



