PERIODIC ARRANGEMENT OF ELEMENTS 167 



respectively, in 6HF, and in 5HF. Only one of the ele- 

 ments of the eighth group appears to be able to replace 

 eight atoms of hydrogen, namely, osmium ; it forms a 

 tetroxide, Os0 4 , thus replacing the eight atoms of hydrogen 

 in four molecules of water, 4H 2 0. But the new gaseous 

 elements of the atmosphere form no compounds, and have 

 no valency, as the power of replacing or combining with 

 hydrogen is termed. They thus form a column by them- 

 selves; and it was interesting to ascertain whether their 

 atomic weights would form a series like those in the other 

 columns. In this case, the atomic weight could not be 

 determined by the usual process of determining the ratio 

 in which the elements combine with hydrogen ; hence a 

 different method was adopted, depending on the known 

 fact that equal numbers of molecules of gases occupy 

 equal volumes under the same conditions of temperature 

 and pressure ; and making use of an argument relating to 

 the number of atoms in such molecules. The atomic 

 weights were : 



These numbers, as will be seen on reference to the table, 

 fit in the eighth column ; the symbols and atomic weights 

 of these gases are printed in italics. They form the initial 

 members of the first and second short series, and of the 

 first, second, and third long series. 



Some doubt exists as to the place to be assigned to 

 hydrogen, the element with lowest atomic weight. Both 

 Mendeleeff and Meyer shirked placing it. It may be that 

 it should be placed at the head of the fluorine column ; 

 but there are equally good, or perhaps better, reasons for 

 believing that it is the first member of the lithium 

 column. 



