SECT. in. ATOMIC WEIGHTS. 99 



seems to be more beyond the power of man than to 

 determine even their relative weights ; yet the definite 

 proportions in which they combine have enabled him 

 to do so. Thus, an atom of oxygen unites with an 

 atom of hydrogen to form water ; but as every drop of 

 water, however small, contains eight parts by weight 

 of oxygen, and one part by weight of hydrogen, it 

 follows that an atom of oxygen is eight times heavier 

 than an atom of hydrogen. Now, since hydrogen 

 gas is the lightest body known, its atom has been 

 assumed as the unit of comparison. Hence, if the 

 unit of hydrogen be represented by 1, that of oxygen 

 may be represented by 8. Again, carbonic acid gas 

 contains six parts by weight of carbon, and eight parts 

 by weight of oxygen, and as an atom of oxygen is eight 

 times heavier than an atom of hydrogen, therefore an 

 atom of carbon is six times heavier than an atom of 

 hydrogen, and consequently may be represented by 6. 

 In this manner the relative weights of many substances 

 have been determined. But the property of isomorphism 

 also affords the means of ascertaining the atomic 

 weights of certain substances with unerring certainty: 

 It is exactly the contrary of dimorphism, for in the 

 latter substances are chemically the same under dif- 

 ferent forms ; whereas isomorphic bodies are chemically 

 different under the same form. Now the peroxide of 

 manganese contains one atom of oxygen for one atom 

 of metal ; but in 100 parts of the protoxide there are 

 21*94 parts of oxygen and 78-06 of manganese. Com- 

 paring these numbers with 8 the atomic weight of 

 oxygen, the result is 28 the weight of an atom of man- 

 ganese. The same number is obtained from two other 

 isomorphic compounds of oxygen and manganese, which 

 proves the accuracy of this result. The atomic weights 

 of many bodies have been determined, of which the 

 following are the most important. 



H2 



