86 



pounds. Then, upon examining the weights of any one element 

 contained in molecular weights of different compounds, we saw 

 (p. 77) that one (the smallest) could be taken as the unit weight, 

 of which the others were multiples by some whole number. Ob- 

 viously then, if the molecular weights are the relative weights of 

 different kinds of molecules, the unit weights of the elements 

 (35.46 for chlorine, etc.) are the relative weights of the atoms of 

 the different elements. The relative weights of the different 

 kinds of atoms, such as 1.008 for hydrogen, 16 for oxygen, 35.46 

 for chlorine, are called the atomic weights of the respective ele- 

 ments. These weights, as we have seen (p. 80), are relative to 

 the weights of the atom of oxygen, when the weight of the latter is 

 taken as 16, and the weight of the molecule of oxygen is taken as 32. 

 A compound molecule may contain one, or more than one, atom 

 of each of the elements forming the compound. In the molecular 

 weight of compounds, weights of elements which are smaller than 

 the atomic weight (p. 77), are never found to occur. This 

 indicates that, in chemical change, fractions of atoms play no 

 part. The name atom (Greek, not cut, or not divided) records 

 this fact. 



The Atomic Hypothesis and Definite Proportions. The 



idea of atoms furnishes at once an explanation of the law of definite 

 proportions. Evidently, every molecule of a given substance must 

 always contain the same numbers and the same kinds of atoms, so 

 that the proportions by weight of the constituents of the com- 

 pound as a whole must be alike in all samples. 



Dulong and Petit's Law. When an element gives no volatile 

 compounds, as is the case with calcium, we can still find the atomic 

 weight. We first find for the element an equivalent weight (p. 

 53). Thus, the weight of calcium which combines with 35.46 

 parts of chlorine is 20 parts. This weight, or some multiple of it 

 by a whole number, does in fact always correctly represent the 



