48 PRINCIPLES OF CHEMISTRY. 



below why we assume, to-day, that the atomic weight of oxygen 

 is 16, and the molecule of water is composed of 2 atoms of 

 hydrogen and 1 of oxygen. 



Another chemical method of determining atomic weights is 

 the replacement of hydrogen atoms in a known substance by 

 the element the atomic weight of which is to be determined. 

 For instance : Hydrochloric acid is composed of one atom of 

 chlorine weighing 35.5, and one atom of hydrogen weighing 1, 

 the molecular weight of hydrochloric acid being 36.5. If in 

 this hydrochloric acid the hydrogen is replaced by some other 

 element, for instance by sodium, we are enabled to determine 

 the atomic weight of sodium by weighing its quantity and that 

 of the liberated hydrogen. Suppose by the action of 36.5 grains 

 of hydrochloric acid on sodium 1 grain of hydrogen be re- 

 placed by 23 grains of sodium, we would say that the atomic 

 weight of sodium is equal to 23. 



The same difficulty exists in this mode of determination of 

 atomic weights which was alluded to above, viz., not knowing 

 whether it was actually one atom of sodium that displaced the 

 one part of hydrogen, a doubt is left whether the determination 

 is absolutely correct. 



Determination of atomic weights by means of specific weights of 

 gases or vapors. It has been stated before that equal volumes 

 of gases contain, under like conditions, the same number of 

 molecules (no matter how small or large the number of atoms 

 within the molecules may be), and that the molecules of ele- 

 ments contain (in most cases) two atoms. These facts give in 

 themselves the necessary data for the determination of atomic 

 weights. 



For instance : If a certain volume of hydrogen is found to 

 weigh 2 grains, and an equal volume of some other gaseous 

 element is found to weigh 71 grains, then the atomic weight of 

 the latter element must be 35.5, because 2 and 71 represent the 

 relative weights of the molecules of the two elements. Each 

 molecule being composed of 2 atoms, these molecular weights 

 have to be divided by 2 in order to find the atomic weights, 

 which are, consequently, 1 and 35.5 respectively. 



In comparing oxygen by this method with hydrogen, it is 

 found that equal volumes of these gases weigh 32 and 2 respec- 



