14 AGRICULTURAL CHEMISTRY 



aration and weighing, the comparative weight, or the 

 number of times heavier or lighter a definite number of 

 molecules is than a similar number of molecules, in other 

 forms of matter, can be accurately determined. While 

 the absolute weight of a molecule or atom cannot be 

 determined, its comparative weight can be. When chlo- 

 rin, for example, combines with hydrogen, it is known 

 that 35.45 times as much, by weight, of chlorin as of 

 hydrogen has entered into combination. Hence the 

 smallest part by weight of chlorin which can combine, 

 must weigh at least 35.45 times as much as the weight of 

 the smallest particle of hydrogen which enters into com- 

 bination. The atomic weights of the more common ele- 

 ments are given in the table on page 1 1 . 



18. Molecular Weights. Since the molecules of com- 

 pounds are composed of a definite number of atoms of 

 elements, and each atom has a definite weight, it neces- 

 sarily follows that a molecule has a definite weight. In 

 the case of water, the formula H 2 O represents one mole- 

 cule of water, composed of two atoms of hydrogen and 

 one of oxygen. As the atoms have definite weights, the 

 weight of the molecule H 2 O is the sum of the weights of 

 the atoms in the molecule. Since hydrogen is taken as 

 the standard and weighs i , and there are two atoms of 

 hydrogen, and one atom of oxygen weighing 16, the 

 weight of the molecule will be 2 -j- 16 or 18 ; that is, the 

 molecule of water, H 2 O, is 18 times heavier than one 

 atom of hydrogen. 



Exercise. Compute the molecular weights of the compounds 

 given in the exercise following the formulas of the compounds, 

 Section 16. 



