Influence of Atomic Weight. 311 



then diminishes in like manner. As a still further illustration 

 of this, Mendeljeff cites the hydrogen-compounds of these ele- 

 ments as far as they are known ; thus : — 



BH 3 *(?) CB: 4 NH 3 OH 2 FH 



SiH 4 PH 3 SH 2 C1H 



Not only does the number of H-atoms vary regularly with 

 the atomic weight, but the stability of these hydrogen-com- 

 pounds under the influence of different agents, as well as their 

 acid characters, and, in fact, all their properties, do likewise. 

 Thus HOl is a powerful and very stable acid ; H 2 S is a weak 

 acid and easily decomposed by.keat; in PH 3 the acid characters 

 are entirely lost and the stability very much diminished, and 

 this still more so in SiH 4 . The physical properties also of 

 these elements vary periodically with their atomic weight, as 

 in the case of the atomic volumes, which are as follows : — 



Li =11-9 Be = 44 B = 41 = 3*6 N=J 0=? F = 



Na=240 Mg = 14-0 A1=1(H) Si = ll-0 P = 16'0 S =16-0 CI =27-0 



The atomic volume diminishes from the beginning to about 

 the middle of each series, and then increases to the end of the 

 series. 



Such relations as have been described above apply not only 

 to the fourteen elements which have been taken in illustration, 

 but to all elements, as will be seen on consulting the following 

 Table (p. 312), in which the elements are arranged in the order 

 of their atomic weights. 



This Table shows that the properties of the elements first 

 change gradually with increasing atomic weight, and then on 

 arriving at a certain point repeat themselves in a new series of 

 elements or a neio period. The change which takes place in 

 the atomicity as the atomic weight increases is a very good 

 example of this. The atomicity either increases up to the 

 middle member of each series, and then diminishes regularly 

 to the end, after which it begins to increase again in exactly 

 the same way in the next series ; or it continues to increase 

 from the first to the last member of the series and then sud- 

 denly falls to unity on commencing with the next series. 

 These facts are rendered evident by the numbers in the second 

 horizontal line of the Table. 



Again, those elements in the same vertical column belong 

 to the same family or group, and of these groups there are 

 eight ; whilst those elements in the same horizontal line belong 

 to the same series, and these series are at present twelve in 

 number. On comparing the elements in any one group, we 

 at once find that they are in each group divisible into two 

 subgroups — and that in such a way that those members of the 

 group belonging to even series are very nearly related to one 

 * Francis Jones, Cliem. Soc. Jo urn. 1879, p. 41. 

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