1813.] Definite Proportions In Chemical Combinations. 41 



oxygen uniting with each would be the same, or the oxygen in 

 the one would be twice, thrice, &c. that in the other. This is 

 another law laid down by Berzelius, and obviously depends upon 

 this fact, that the two bodies must either unite atom to atom, or 

 a certain number of atoms of the one must combine with one 

 atom of the other. t ■ 



This law might be applied successfully to determine which of 

 the metallic alloys are chemical combinations, and which are 

 mechanical mixtures. For example, there can be no doubt that 

 copper and zinc combine chemically. Now from the following 

 table it will appear that the weight of an atom of these metals is 

 as follows: — 



Copper 8-000 



Zinc '1-315 



Therefore, if they unite atom to atom, brass ought to be a 

 compound of 100 copper and 53-93 zinc. Now if any person 

 will be at the trouble to analyse brass, he will find that this is 

 very nearly the proportion of the ingredients. 



In like" manner bell-metal seems to be a compound of 5 

 atoms of copper to 1 atom of tin ; and the metal for mirrors, of 

 4 atoms of copper and 1 atom of tin. If mercury unites atom 

 to atom with tin, it ought to dissolve somewhat less than half 

 its weight of that metal. And if the same law holds with zinc, 

 it ought to dissolve about £th of its weight of that metal. 



III. Relative Weight of the Atoms of different Substances 

 determined from Chemical Analysis. 

 Before we can draw up a table of the relative weights of the 

 atoms of bodies, we must fix upon some one whose atom shall 

 be represented by unity. Mr. Dalton has made choice of 

 hydrogen for that purpose, because it is the lightest of all known 

 bodies. Sir Humphry Davy has followed his example ; but he 

 has doubled the weight of an atom of oxygen, and consequently 

 of all other bodies, by the arbitrary supposition that water is 

 composed of two atoms of hydrogen and one of oxygen. Dr. 

 \\ oUaston and Professor Berzelius have both proposed the atom 

 of oxygen as the most convenient unit : nor can there be any 

 hesitation in embracing their plan. Oxygen is in fact the 

 substance by means of which the weight of the atoms of almost 

 all other bodies is determined. It enters into a much greater 

 number of combinations than any other known body; heme the 

 great advantage attending a convenient number for that body to 

 the practical chemist. 



It would remove a great deal of confusion, which is at pre- 



nt very conspicuous in this department <>f the science, if 



..lists would agree to represent thu weight of the atoms by 



