THE PERIODIC LAW 



145 



dulatory, with well-defined maxima and minima, which occur 

 at regular intervals. The curves for most of the other prop- 

 erties which are capable of precise measurement are found to 

 have a similar character; the maxima and minima, of course, 

 do not always coincide with the same elements in one curve 

 as in another, but the elements which occupy similar positions 

 on one curve are also found to be similarly located on another. 

 It is especially noticeable, moreover, that such curves indi- 

 cate a relationship between the groups of elements, as well as 

 between the individual elements of each single group. Thus 

 the properties of the alkaline-earth metals are always found 

 to be intermediate between those of the alkalies and those 

 of the aluminum group. Breaks in the continuity of the 

 curves indicate lack of sufficient experimental data. - t .... 



Centigrade 



70 80 90 100 110 120 130 140 150 160 170 "180 190 200 21Q- 

 ATOMIC WBIAHTS 



The arrangement of the elements, as shown in the accom- 

 panying table (p. 148), is the one generally adopted at pre- 

 sent, and includes all the well-known elements. An asterisk 

 marks the elements discovered since 1869. Hydrogen occupies 

 a unique position, and is generally omitted from the classifi- 

 cation. Argon, helium, neon, and krypton cannot be properly 

 included as yet, because their chemical behavior is still un- 

 known. The vertical columns include the elements most 

 closely associated with one another, and are known as Groups 

 I, II, etc. ; horizontally, we have the Series 1,2,3, etc., in which 

 the similarities are not great, excepting that a parallelism 

 exists between the elements of one series as compared 

 with those of another. The elements in odd-numbered series 

 bear a closer resemblance to one another than they do to the 



