276 SMITH'S INTERMEDIATE CHEMISTRY 



LiCl G1C1 2 BC1 3 CC1 4 ; CH 4 NH 3 OH 2 FH 

 Li 2 G10 B 2 3 C0 2 N 2 5 



Nad MgCl 2 A1C1 3 SiCl 4 ; SiH 4 PH 3 SH 2 C1H 

 Na 2 O MgO A1 2 3 Si0 2 P 2 5 S0 3 C1 2 O 7 



Thus the valence towards chlorine and hydrogen ascends to four 

 and then reverts to one in each octave. The highest valence, 

 shown in oxygen compounds, ascends from lithium to nitrogen 

 with values one to five, and then fails because compounds are 

 lacking. In the second octave, however, it goes up continuously 

 from one to seven. 



Again, the specific gravities of the elements in the second series, 

 using the data for red phosphorus and liquid chlorine, are : 



Na 0.97, Mg 1.75, Al 2.67, Si 2.49, P 2.14, S 2.06, Cl 1.33. 



Mendelejeff's Scheme. In 1869 Mendelejeff published an 

 important contribution towards adjusting the difficulty which the 

 elements following chlorine presented, and developed the whole 

 conception so completely that the resulting system of classifica- 

 tion has been connected with his name ever since. The table 

 on page 278, in which the atomic weights are expressed in round 

 numbers, is a modification of one of Mendelejeff's, extended to 

 include elements more recently discovered. 



The chief change made by Mendelejeff from the arrange- 

 ment in 'simple octaves is that the third series, beginning 

 with potassium, is made to furnish material for two octaves, potas- 

 sium to manganese and copper to bromine, and is called a long 

 series. The valences fall in with this plan fairly well. Copper, 

 while usually bivalent, forms also a series of compounds in which 

 it is univalent. Iron, cobalt, and nickel fall between the two 

 octaves, and cannot be accommodated in either. 



Every long series contains three elements of this character, 

 closely resembling one another. As will be seen from the table,, 

 these transition elements, as they are called, may be placed together 



