106 PRINCIPLES OF CHEMISTRY. 



that of the liberated hydrogen. Suppose that by the action of 36.2 

 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 

 found to weigh 71 grammes, 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 com- 

 posed 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 by this method oxygen with hydrogen, it is found 

 that equal volumes of these gases weigh 32 and 2 respectively, that 

 the atomic weight of oxygen is consequently 16, and not 8, as deter- 

 mined by chemical methods. 



This mode of determining atomic weights may be applied to all 

 elements which are gases or which may without decomposition be 

 converted into gas. There are, however, elements which cannot be 

 volatilized, and in this case it becomes necessary to determine the 

 specific gravity of some gaseous compound of the element. The 

 element carbon itself has never been volatilized, but we know many 

 of its volatile compounds, and these may be used in the determina- 

 tion of its atomic weight. 



Determination of atomic weights by specific heat. Specific 

 heat has been stated to be the quantity of heat required to raise the 

 temperature of a given weight of any substance a given number of 

 degrees, as compared with the quantity of heat required to raise the 

 temperature of the same weight of water the same number of degrees. 



