APPLICATION OF THE MOLECULAR HYPOTHESIS 



87 



amount of calcium combined with atomic weights of elements 

 whose atomic weights are known. Hence 20, or some multiple of 

 20 is the required atomic weight. Dulong and Petit (1819) dis- 

 covered a rule by which the correctness of such an atomic weight 

 could be checked. When the atomic weight of an element is 

 multiplied by the specific heat of the element in the solid form, 

 the product is roughly 6.4. Thus, the known atomic weight of 

 magnesium is 24.3, its specific heat 0.245, and the product is 

 5.95. Again, the known atomic weight of mercury is 200.6, and 

 its specific heat 0.0335, and the product 6.7. Now the specific 

 heat of calcium is 0.170, and the product 20 X 0.170 is equal to 

 3.4. But 2 X 20 X 0.170 = 6.8. The value 2 X 20, or 40 for 

 the atomic weight is therefore the correct one. The following 

 table contains additional illustrations of this law: 



Another way of expressing this law will give it greater chemical 

 significance. The specific heats are the amounts of heat required 

 to raise one gram, that is one physical unit, of each element through 

 one degree. When we multiply this by the atomic weight, we 

 obtain the amount of heat required to raise one gram-atomic 

 weight of the element, that is, one chemical unit, through one 

 degree. The values of this product are approximately equal. 

 Since there are equal numbers of atoms in one gram-atomic weight 

 of each element, it follows that: Equal amounts of heat raise 

 equal number of atoms of all elements in the solid form through 

 equal intervals of temperature. 



