ATOMIC THEORY. 421 



Dalton ran thus. If in the chemical union of any two simple 

 bodies we suppose a single atom of each to form the com- 

 bination, then if by analysis we obtain the precise relative 

 weights of the two in the compound, we gain in effect the 

 relative weight of the atoms thus combined. Take the case 

 of water in illustration. The uniform and peculiar characters 

 of this wonderful fluid suggested the inference which Dalton 

 adopted, that it is formed by the union of a single particle 

 of hydrogen to one pf oxygen. Analysis proving the relative 

 weight of the two elements in water to be as 1 to 8, the con- 

 clusion was immediate, that the same ratio represented the 

 weight of the atoms of hydrogen and oxygen respectively in 

 this fluid. 



But more was needed yet to justify this conclusion, and 

 some third body required to be brought into the chain of 

 evidence. Taking carbon as the instance, and examining 

 carbonic oxide gas, the simplest form of its combination with 

 oxygen, we find the relative weights of the two elements in 

 this compound to be 6 of carbon to 8 of oxygen ; whence it 

 is inferred that an atom of each enters into the compound, and 

 that the relative weights of these atoms are as the numbers 

 given. Thus, then, we have obtained three proportions of 

 weight: 1 for hydrogen, 6 for carbon, and 8 for oxygen. 

 Now mark the beauty of the evidence. If these three num- 

 bers be correct, we ought to find them recurring, either in 

 simple or multiple form, in all the compounds of carbon and 

 hydrogen, thus completing the circle of proof. And such is 

 actually the case. The details of this demonstration belong 

 to Chemistry ; but the more they are extended to different 

 substances, simple or compound, the more complete and 

 striking does the evidence become. 



Stopping a moment here, let us see precisely how we are 

 to define the knowledge thus far gained. One essential point 

 will already be understood, viz. that in speaking of atomic 



