84 AN INTRODUCTION TO SCIENCE 



(3) Hydrogen and the gas, helium (At. W. 4), recently 

 discovered by Professor Ramsay, stand alone in the first 

 series, no other member of this series being known. 



(4) There were several gaps in the table, of which some 

 have since been filled up. 



(5) The differences between the atomic weights of the 

 several members of each series or of each group are only 

 approximately constant. 



Despite its want of arithmetical rigidity, there can be 

 no doubt that Mendele*efs classification is based upon 

 natural laws. The elements which he arranged in groups 

 resemble each other in properties ; their differences are 

 differences in degree. The elements in the series differ 

 from one another in properties, and the amount of their 

 differences increases progressively from the first to the 

 seventh or eighth member. Their properties therefore vary 

 in kind. 



Take, as an example of properties, the tendency to form 

 oxides. Most of the elements form more than one oxide, 

 but for each of them there is one oxide which chemists 

 regard as characteristic. If R stands for any element, the 

 characteristic oxide of group I. is R 2 O ; of II., R 2 O 2 ; of 

 VII., R 2 O 7 . 



If we compare the hydrogen-holding power of the 

 elements with their oxygen-holding power, we find that 

 their capacities in this respect are reversed. The hydrides 

 of group VII. have the formula RH ; of VI., RHo ; cf V., 

 RH 3 ; and of IV., RH 4 . The justification of the octave 

 arrangement is shown very clearly by these two sets of 

 compounds. No single atom of any element can, so far as 

 is known, combine with more than four atoms of oxygen, 

 or. with more than four atoms of hydrogen ; but if its maximum 

 hydrogen-holding power and its maximum oxygen-holding 

 power are considered together, it is found that the number 

 of atoms of hydrogen which it can hold, plus twice the 

 number of atoms of oxygen (because O is divalent), always 



