ATOMIC THEORY. 121 



size, form, or even actual weight of the atoms of elementary 

 bodies, or as to the mode in which they are grouped or arranged 

 in compounds. All that is known or likely ever to be known 

 respecting them is their relative weight. The atom of oxygen 

 is eight times heavier than that of hydrogen, or they are to each 

 other as 100 to 12.5, but their actual weights are undetermined. 

 To afford the means of expressing the relative weights of these 

 and other atoms, a number which is entirely arbitrary is assigned 

 to one of them, namely J 00 to the atom of oxygen, and then 

 the weight of the atom of hydrogen can be said to be 12.5, of 

 nitrogen l77> of carbon 76.4, of sulphur 201, and of lead 1294.5. 

 A single atom of water contains one atom of oxygen, (100) and 

 one of hydrogen, (12.5) and must therefore weigh 112.5; an 

 atom of oxide of lead contains one atom of oxygen and one of 

 lead, which weigh together 1394.5; an atom of sulphuric acid, 

 one atom of sulphur and three atoms of oxygen, which weigh 

 together 501 ; and an atom of sulphate of lead, including one of 

 each of the preceding compound atoms must weigh 1294.5 + 

 501, or 1795.5. 



The equivalent quantities being now represented by atoms, 

 it necessarily follows that bodies can combine in these quantities 

 or multiples of them only, and not in intermediate proportions, 

 for atoms do not admit of division. In a series of several com- 

 pounds of the same elements, such as the oxides of nitrogen, 

 which was formerly referred to in illustration of combination in 

 multiple proportions, (page 109) one atom of nitrogen combines 

 with one, two, three, four and five atoms of oxygen, and a simple 

 ratio between the quantities of oxygen in these compounds is the 

 consequence. The equivalent of a compound body also is the 

 sum of the equivalents of its constituents, for the weight of a 

 compound atom is the weight of its constituent atoms. 



By the juxtaposition, separation, and exchange of one atom 

 for another in compounds, all kinds of combination and de- 

 composition in equivalent quantities may be produced, while 

 the substitution of ponderable masses for the abstract idea of 

 equivalents renders the whole changes most readily conceivable. 



This theory being adopted as a useful, while it is at the 

 same time, a highly probable representation of the laws of 

 combination, its terms atom and atomic weight may be used 

 as synonymous with equivalent, equivalent quantity and com- 

 bining proportion. 



