DETERMINATION OF ATOMIC WEIGHTS. 49 



atoms. If 56 represented 2 atoms, the atomic weight would be but 

 28; if 4 atoms, 14. 



As this mode of determination gives no clue to the number of 

 atoms present in the molecule, the results obtained are liable to be 

 incorrect. In fact, the atomic weights of a number of elements had 

 originally been determined incorrectly by using the above or similar 

 methods, and many of these old atomic weights had to be changed 

 (generally doubled) in order to obtain the correct numbers. 



Thus, in examining water, it was found that it contained 8 parts 

 by weight of oxygen to 1 part of hydrogen, and the conclusion was 

 drawn that the atomic weight of oxygen was 8, and that the molecule 

 of water was formed by the union of one atom of hydrogen and one 

 atom of oxygen. It will be demonstrated below why we assume to- 

 day that the atomic weight of oxygen is 16, and that 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 : Hy- 

 drochloric acid is composed of one atom of chlorine weighing 35.4, 

 and one atom of hydrogen weighing 1, the molecular weight of hy- 

 drochloric acid being 36.4. If in this acid the hydrogen be replaced 

 by some other element, for instance by sodium, we are enabled to 

 determine the atomic \veight of sodium by weighing its quantity and 

 that of the liberated hydrogen. Suppose that by the action of 36.4 

 grammes of hydrochloric acid on sodium, 1 gramme of hydrogen 

 was replaced by 23 grammes of sodium. In that case we would say 

 that the atomic weight of sodium is equal to 23. 



The difficulty which was alluded to above exists also in this mode 

 of determination of atomic weights, viz., not knowing whether it 

 was actually one atom of sodium that replaced the one part of hy- 

 drogen, a doubt is left as to whether or not the determination is 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 few or many the atoms within the molecules may be), 

 and that the molecules of elements contain (in most cases) two atoms. 

 These facts give in themselves the necessary data for the determina- 

 tion of atomic weights. 



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

 2 grammes, and an equal volume of some other gaseous element is 



4 



