GENERAL REMARKS REGARDING ELEMENTS. 65 



highly characteristic feature, which consists in the fact that the corre- 

 sponding members of the even (2, 4, 6, etc.) periods and of the uneven 

 (3, 5, 7, etc.) periods resemble each other more closely than the mem- 

 bers of the even periods resemble those of the uneven periods. Thus 

 the metals calcium, strontium, and barium, of the even periods, 4, 6, 

 and 8, resemble each other more closely than they resemble the metals 

 magnesium, zinc, and cadmium, of the uneven periods, 3, 5, and 7, the 

 latter metals again resembling each other greatly in many respects. 



It is for this reason that in the table the elements belonging to one 

 group are not placed exactly underneath each other, but are divided 

 into two lines containing the members of even and uneven periods 

 separately, whereby the elements resembling each other most are 

 made to stand together. 



In arranging the elements by the method indicated, it was found 

 that the elements mentioned in group VIII. could not be placed in 

 any of the 12 small periods, but that they had to be kept separately 

 in a group by themselves, three of these metals always forming an 

 intermediate series following the even periods 4, 6, and 10. 



An uneven and even series, together with an intermediate series, 

 form a large period, the number of elements contained in a complete 

 large period being, therefore, 7 + 7 + 3 = 17. 



An apparently objectionable feature is the incompleteness of the 

 table, many places being left blank ; but it is this very point which 

 renders the table so highly interesting and valuable. 



Mendelejeff, in arranging his scheme, claimed that the places left 

 blank belonged to elements not yet discovered, and he predicted not 

 only the existence of these as yet missing elements, but also described 

 their properties. Fortunately his predictions have, in at least three 

 cases, been verified, three of the missing elements having since been 

 discovered, and named, scandium, gallium, and germanium. These 

 elements not only fitted in the previously blank spaces by virtue of 

 their atomic weights, but their general properties also assigned to 

 them the places which they now occupy. 



Physical properties of elements. Most elements are, at the 

 ordinary temperature, solid substances, two are liquids (bromine and 

 mercury), five are gases (oxygen, hydrogen, nitrogen, chlorine, and 

 fluorine). Most of the solid elements may be converted into liquids 

 and gases by the action of heat. Some solid elements, however, have 

 so far resisted all attempts to change their state of aggregation, as, for 

 instance, carbon. 



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