DETERMINATION OF ATOMIC WEIGHTS. 47 



60. Suppose a certain volume of hydrogen to weigh 20 grains, how 

 much will an equal volume of oxygen and how much an equal volume 

 of hydrochloric acid gas weigh, provided pressure and temperature be 

 the same ? 



7. DETERMINATION OF ATOMIC WEIGHTS. 



Determination of atomic weights by chemical decomposition. 

 The great difficulties originally encountered in the determina- 

 tion of atomic weights cannot be described here, and the present 

 methods only, of which there are principally three, will be con- 

 sidered. These methods depend either on chemical action or 

 physical properties. 



One of the chemical methods used for the determination of 

 atomic weights has been stated before in describing the decom- 

 position of the red oxide of mercury by heat. The principle 

 of this method is the determination of the proportions by weight 

 in which the element, the atomic weight of which is unknown, 

 combines with an element the atomic weight of which is known. 

 For instance : If in decomposing a substance we find it to con- 

 tain in 72 parts by weight 16 parts by weight of oxygen and 56 

 parts by weight of another element, we have a right to assume 

 the atomic weight of this second element to be 56, provided, 

 however, that the compound is actually formed by the union of 

 one atom of oxygen and one atom of the other element. These 

 56 parts by weight might, however, represent 2 or 3 or more 

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

 but 28 ; if 4 atoms, 14. 



As this mode of determination gives no clew 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 ele- 

 ments had originally been determined incorrectly by using the 

 above or similar methods, and many of these old atomic weights 

 had to be corrected (generally doubled) in order to obtain the 

 correct numbers. 



Thus, in examining water, it was found that it contains 8 

 parts by weight of oxygen to every 1 part of hydrogen, and the 

 conclusion was drawn that the atomic weight of oxygen was 8, 

 the molecule of water being formed by the union of one atom 

 of hydrogen and one atom of oxygen. It will be demonstrated 



