24 ORGANIC EVOLUTION CONSIDERED 



composition of water can vary by a single atom of 

 either of its elements. He feels certain that the 

 rule is infallible that each molecule of water must 

 contain two atoms of hydrogen and one atom of 

 oxygen, and that it can have no other composition. 

 The same principle holds good of every other com- 

 pound. 



Again, it often occurs that the same elements 

 unite in different proportions to form different com- 

 pounds, and, in these cases, they unite according to 

 the law of " multiple proportions," namely, that 

 when two elements, A and B, unite in more than one 

 proportion, if we take quantities of the compounds 

 which contain the same amount of A, the quantities 

 of B will bear a very simple relation to each other. 

 For example, take the compounds of nitrogen and 

 oxygen represented by the following five formulas, 

 N 3 O, N 2 2 , N 3 3 , N 2 Oi and N 2 6 . It will be 

 noticed that while each compound has two atoms of 

 nitrogen in its molecule, the amounts of oxygen are 

 simple multiples of the quantity in the first com- 

 pound. 



The formula for water is H2 O, but there is another 

 compound of oxygen and hydrogen, H2 O2, known as 

 hydrogen dioxide, which is a very active oxydizing 

 agent. 



Chemists have determined with great exactness 

 the relative weights of atoms. I shall not describe 

 the methods by which this has been done. Hydro- 

 gen being the lightest known substance, its atomic 

 weight is taken as unity. The atomic weights of the 

 seventy elements range from 1 to 239, the latter being 

 the atomic weight of uranium. 



It is a remarkable fact that when elements combine 

 with each other, their amounts are represented by 

 their atomic weights or by some multiple of these 



